Question

The value of y varies directly with x, and y = 12 when x = 8.
Find y when x = 15.

Answer 1

y = kx (Direct variation formula)
Now sub 12 in for y and 8 in for x
12 = k(8)
12/8 = k
3/2 = k
now , use the same formula, and sub 15 in for x
y = 3/2(15)
y = 45/2 or 22 1/2 or 22.5

or this way....
8/12 = 15/x (set it up as a proportion)
cross multiply..
(8)(x) = (12)(15)
8x = 180
x = 180/8
x = 22 1/2 or 22.5