how do I find this?
There are 2 ways to complete the problem. The easiest is the same as the previous answers, cross-multiplication.
3 = 5
21 = x
This is a great, quick method, however I have found that students who only learn this method make many mistakes when the questions get more complicated. Another way to solve this problem, and avoid mistakes is to take it step by step.
3 = 5
First flip it so that the variable is on top
x = 7
Then get x by itself, remember x is always a loner, he doesn't like anyone near him. To do this multiply both sides by 3 ( is being divided by 3, so do the opposite operation).
3x = (3)7
The 3's cancel, leaving x. Remember that any number can always have a 1 as it's denominator.
x = 3 . 7
Multiply straight across
x = 21
Yes, this method is a little more work, but it will stop mistakes in the future.
Hope this helps and good luck in the new school year!