Question

How could you use cross products to check the solution to this proportion `12/30=5/x`?

Answer 1

`12 xx12.5 = 30xx5`
`" "150=150`

Explanation:

You will have to have a solution first ..

Cross multiplying is a way of finding the solution, but we will use that to check later. Let's use another method to find `x`

`12/30 = 5/x`

Notice that `12 div 2.4 =5`

`(12 div 2.4)/(30 div 2.4) = 5/12.5" " rarr x = 12.5`

To check whether this is correct: cross-multiply...

`color(lime)(12)/color(blue)(30) = color(blue)(5)/color(lime)(12.5)`

Is `color(lime)(12 xx 12.5) = color(blue)(30xx5)?`

We find: `color(lime)(150) = color(blue)(150)`

Therefore we have the correct solution for `x`

This is true for any equivalent fractions:

`7/5 = 21/15" " 4/11 = 20/55`

`105 =105" "220 = 220`