Question

If y varies directly with x, and If y = 12 when x = 15, how do you find x when y = 21?

Answer 1

`x=26 1/4`

Explanation:

Use ratios such that `" " (x_1)/(y_1) = (x_2)/(y_2)`

Also note that the condition of 'TRUE' also holds for

`" "(y_1)/(x_1)=(y_2)/(x_2)`

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Need to find `x_2` so it works much easier if you use `x` as the numerator, giving:

`" " 15/12 = x/21`

Multiply both sides by 21 and we have:

`" "21xx15/12= x xx21/21`

But`" " 21/21 = 1` giving:

`" "x=(21xx15)/12 = 26 1/4 " "` which in decimal form is 26.25