find the required value by setting up the general equation and then evaluating.
find y when x=10 if y varies directly as x and y=20 when x=8
thanks!
When y varies directly as x, we have the equation $\displaystyle y = kx$. The first thing to do is solve for k. Substituting in y = 20 and x = 8 we have $\displaystyle 20 = 8k$, or $\displaystyle k = \frac{20}{8} = \frac{5}{2}$.
Given that $\displaystyle y = \frac{5}{2}x$, can you determine y when x = 10?
Icemanfan, hope you don't mind me just adding something.
mamajen, In the other question you asked, I said what "proportional" mean and told you about putting the k there and then working out what k was. Well, "varies directly as" just means "is proportional to", so the two questions are quite similar.