Question

If y varies directly as x and y = -18 and x = 3, find x when y = 30.

Answer 1

`y \: varies \: directly \: as \: x \: mean \\ \: y \: increase \: or \: decrease \: with \: x \\ y = - 18 \: \: \: \: x = 3 \\ y = 30 \: \: \: x = ? \\ - 18 ? = 90 \\ ? = - 5 = x`

Answer 2

x = - 5

Explanation:

Given y varies directly as x then the equation relating them is

y = kx ← k is the constant of variation

To find k use the condition y = - 18 when x = 3 , then

- 18 = 3k ( divide both sides by 3 )

- 6 = k

y = -6x ← equation of variation

When y = 30 , then

30 = - 6x ( divide both sides by - 6 )

- 5 = x