Question

Y and x have a proportional relationship, and y = 15 when x = 3. What is the value of x when y = 4?

Answer 1

y = kx
k = y/x = 15/3 = 5
when y = 4
5 = 4/x
x = 4/5

answer
when y = 4 , x = 4/5

Answer 2

When two values vary or have a proportional relationship there is always some value "k" you need to solve for first. It is a constant multiplier k.

proportional relationship: y = kx
Use the given values (3,15) to find k.
15 = 3k
15/3 = k
5 = k

Equation is now: y = 5x
when y = 4
4 = 5x
4/5 = x