Question

The value of X and Y vary directly, and when X= 4 Y equals -10. Find the value of Y when X equals 10.

Answer 1

y = - 25

since x varies directly with y then we can express the relationship as

x = ky ← k is the constant of variation

to find k use the given condition y = - 10 when x = 4

k = $\frac{x}{y}$ = $\frac{4}{-10}$ = - $\frac{2}{5}$

hence x = - $\frac{2}{5}$ y

when x = 10

10 = - $\frac{2}{5}$ y

multiply both sides by - 5 and divide both sides by 2 )

- 50 =  2y ⇒ y = - 25