Hello, Shelby,
When I see "varies directly as x," I envision a general equation of the format y = mx. As you increase x, y will also increase. That;s the "varies directly" part. What we don't know is HOW much y will change when x goes up. The m is the value that tells us how much y will change when x goes up or down.
If y increases by 1 when x is increased by 1, then m=1. But if x goes up by 2, then m = 2. and so forth.
We are told that for x=20, y is 12. Let's find the value of m with this input:
y = mx
12 = m(20)
m = 3/5
So we have y = (3/5)x
If x is 15, we would have:
y = (3/5)(15)
y = 9
Bob
