Answer:
y = 4x
Step-by-step explanation:
When we say that x and y "vary directly", we mean that they have the relationship: y = kx, where k is a "constant of proportionality".
Here, we want to know what k is, so we plug 4 in for x and 16 in for y:
y = kx
16 = k * 4
k = 16/4 = 4
Thus, our equation is y = 4x.
