Question

Suppose y varies directly as x and y equals 21 when x = 3 find x when y = 42

Answer 1

y α x

y = cx where c = constant of proportionality

cx = y

c = y/x

y = 21, x = 3, y = 42, x = ?

y/x = c

21/3 = 42/x

7 = 42/x

x = 42/7

x = 6

so x = 6

Hope this explains it.

Answer 2

y = kx find k

21 = k*3
k = 7

Equation:
y = 7x
Find x when y = 42
42 = 7x
x = 42/7
x = 6

answer: x = 6 when y = 42