Explain How To Do Criss Cross Multiplying in Fraction?

Question

Examples also please!

Also how do you do it with algerbra like this:

5/x = 4/5

Answer 1

That's how you solve proportions.(two fractions, with an equal sign in between them.)
When multiplying fractions you don't "criss cross" or "cross multiply", that's a common mistake. You multiply the tops for the top of the answer, and you multiply the bottoms for the bottom of the answer.
For example, if you want to multiply 1/2 times 3/4 you don't do (1x4)/(2x3), you do (1x3)/(2x4). I've seen people insist on "cross multiplying" when multiplying fractions, and wonder why they always get it wrong.
"Cross multiplying" or "Criss cross" is a shortcut for solving proportions.
For example,
what is 3/4 of 78?
think
3/4 = x/78
I know, it's hard to see what's on top and what's on bottom here, that's a limitation of this online text editor, but if these fractions were written out in the usual way, you'd do, like I explain it to kids, "upstairs times downstairs, divided by the leftover, gives you the answer"
So, "cross multiplying", (3x78)/4 = x = 58.5
It's also useful for comverting to common denominators, which is really solving a proportion.
For more info and examples see
http://www.math.com/school/subject1/lessons/S1U2L2GL.html
http://amby.com/educate/math/4-2_prop.html
http://www.lessontutor.com/sv1.html

Answer 2

Cross Multiplication Examples

Answer 3

Okay, so basically what you want to do is take a fraction and multiply tops to bottoms on the opposite of the equal sign (or inequal sigs such as < or >). Ex. 2/3=4/5, becomes (2x5) = (4x3). Get it?

Answer 4

if a/b = c/d (b,d <>0) then ad = bc
5/x = 4/5
=> 4x = 25 => x = 25/4