I have to remember how to cross multiply.

User in Columbus, OH
June 25, 2018

I've always been an advanced student. Today my teacher threw a bombshell at me and gave me a test to do about cross multiplying. I haven't done that since middle school, can anyone refresh my memory? its sad I am a high schooler asking how to cross multiply, But honestly: who remembers something the haven't done in 5 years?

User in Tracy, CA
Jan. 12, 2016

A nice, easy way to remember when to cross multiply is an equal sign between fractions. It's like when you see something similar to this: (50/100=?/2) Since there is an equal sign in between the two fractions, all you do is multiply 50 with 2 and 100 with ?. That will give you: (100=100?) After that just solve by dividing. So, pretty much, all you really need to look for is that sneaky equal sign (=). I hope this helped :)

Hahah I can show you with a pen and paper. Like up the equation draw and draw an x over the equal sign. pull the top of the left and bottom of right, or a dash of the x, to the one side of a new equation (multiply/divide/add/subtract if you must). Do the same with the other dash. Put the same grouping on one side (group the X's together), then simplify. Much easier if I write all this on paper.

Nila in San Jose, CA
June 29, 2018

yes, you have to remember to advance.

Cross multiplication is a process by which one simplifies an equation involving two fractions. Example: A/B = X/Y 'cross' multiply A*Y and B*X. Now you have AY = BX. That's pretty much it... More complex example: (5x - 2)/2 = (x + 4)/3 'cross' multiply 3*(5x - 2) and 2*(x + 4), which becomes: 15x - 6 = 2x + 8

Say you have 3/6 = 1/2. you would multiply the 6 times the 1, and the 3 times the 2. If you do it out on paper, your multiplying in the shape of a cross! Your next line would then be 6(1)=3(2), which both equal 6! If you use variables, this method could be used to solve for it. Good luck!

John in Spartanburg, SC
June 15, 2018

Going back to the beginning of math and assisting in showing multiplication principles.

Janalie in Chicago, IL
May 28, 2018

That's a very fair point. Cross-multiplication is an easy topic to forget; however, it's also a quick topic to pick up, once you remember how to do it: take two fractions that are equal to each other. Multiply the denominator of one by the numerator of the other. Those new equations are now equal to each other. From there, you can do simple algebra to figure out the variable. Hope that helps!

Isabella in Portland, OR
March 10, 2018

ok so if you have A/B x C/D (pretend those are fractions) you multiply A x D and B x C and end up with AD = BC and solve that. for example 10/2 x N/5 is 10 x 5 and 2 x N so you then have 50 = 2N and you solve for N if my explanation doesnt have sense here is a wikihow page that explains how it works. https://www.wikihow.com/Cross-...

Not sad at all! It's difficult to retain information we don't use regularly- most of us don't use that method because of computers/calculators. Try to find a YouTube tutorial. There are some good ones out there that can give you a visual.

Dena in Studio City, CA
Oct. 18, 2017

Is it with a single variable? If so: multiply the numerator of the left-hand fraction by the denominator of the right-hand fraction. Let's say you're working with the equation 2/x = 10/13. Now, multiply 2 * 13.

I suggest a quick youtube video. No shame in brushing up on skills, we all forget things from time to time :)

User in New York, NY
Sept. 21, 2017

Take a look at Kahn Academy, they have great math tutorials. Don't beat yourself up, forgetting is part of having an efficient brain, and learning a second time helps you remember the information better in the long term.

Dianna in Lockport, LA
Aug. 29, 2017

Numerator multiplied by opposite denominator. Repeat. You're done. a c - x - a x d b x c b d

Alexandra in Plano, TX
June 16, 2017

Just multiply the denominator of one side by the numerator of the opposite side and vice versa. Then set the two values equal to each other. For example, if you had 5/6=2/3 you would multiply 5 by 3 and 6 by 2, leaving you with 15=12. (The 2 fractions are obviously not actually equal, so that's why the values aren't)

a/b = c/d. First step is to multiply (a x d). Second you multiply (b x c). Next you take the answer from (a x d) and divide the answer from (b x c).

User in Las Vegas, NV
Nov. 24, 2016

Go back and take a look. It sounds like you don't like fraction and forgot about them after you dealt with fraction 5 yrs ago. Just because you haven't done them for 5 yrs, it does not conclude that you are completely out of it. You will face more fractions until you graduate. So my advice is this: watch youtube everyday until it sinks in. Or visit one of math websites and revisit how to deal with fractions (add, subtract, multiply, divide, cross-multiply, equivalent fractions). Write them down and practice many problems as you can. It feels like you have been using the calculator for your entire time with math. Do not use the calculator.

Hey, Most people don't remember things that haven't practiced in a while...don't feel bad! If you would like to work with a tutor for a hour to help you remember this skill, please reach out to me. Thanks, Deju Green Education Consultant www.edgetutoringservices.com

User in Carnegie, PA
Oct. 2, 2016

User in Haughton, LA
Sept. 6, 2016

Sure. If you are dealing with fractions then you cross multiply when dealing with dividing them. There is a saying: Multiplying fractions is no problem - top times top/bottom times bottom. Dividing fractions don't ask why. Flip the second and multiply. This is the same as cross multiplying.

User in Jamesburg, NJ
June 10, 2016

Since you are an advanced student, I wouldn't worry about one bombed test. Ask your teacher for help after school so that you are prepared in the future.

I am assuming you are talking about fractions? It's just basic beginning algebra. Such as x over 16 = 4 over x. I can't write expressions on this post, but it would be x squared = 4x16. Calculate x squared = 64 Then solve, x=8, or -8. This website will show how to do this very clearly (unlike I can by typing in this box) :) https://www.mathsisfun.com There are a couple of ways to do this, and they usually have several examples. Good luck! :)

Super simple. 10/50=x/4 Cross multiply 10*4=50*x 40=50x 40/50=x 0.8=x That is all.

Also, if you're someone who enjoys finding little things that'll remind you what to do, just remember that cross multiplying makes a bow tie. If you draw a line from the bottom number to the diagonal top number on the other side of the equal sign and then draw a line straight down from that number to show where the answer goes, and then repeat for the other side, it makes a bow tie when the lines overlap.

User in Laurel, MD
July 27, 2016

User in Tustin, CA
Aug. 9, 2016

Cross multiplying is when you have a fraction let's say 2/5 times 3/5 and you have to multiply them together so then what you do is draw a diagonal line to help, from the two to the five and the three to the other five and then multiply those numbers. Then you would have 10 and 15 so then you add those together and then you get twenty five over one as the answer if I am remembering this correctly.

User in Mulvane, KS
April 18, 2016

Cross multiplication is when you multiply two fractions diagonally across.

If you have to cross multiply. 1\2  and  3\4 , remember the top numbers of the fractions are numerators and the bottom numbers are denominators.

First you multiply 1 and 4 which will give you your new denominator, then you multiply 2 and 3 to get the new numerator.

So, in this case the answer is 6\4 which is an improper fraction, so you simplify the fraction making the answer to be 1 1\2. I hope that helps you!  Good luck!

Frances in Bronx, NY
July 13, 2018

I personally google every question I don't know how to answer, heres something that will help from google. (Cross multiplying proportions is a straightforward process that is the same as cross multiplying fractions. Cross multiply by multiplying a numerator by the other side's denominator. The following video shows some examples of cross multiplication.) I am an english tutor. Math is not my strong suit.

User in Columbus, OH
Jan. 12, 2016

Thanks guys, I didn't know I would ever be using cross multiplication in a Exam in High school, thank God I am homeschooled.