   SEARCH HOME Math Central Quandaries & Queries  Question from Ben, a student: Solve the formula for K: C=30d+0.10k Hi Ben.

When we say "solve for K" we mean to isolate K by itself on one side of the equal sign.

You can do this by "moving" things from one side to the other using mathematical rules.

The most important principle is this:
whatever you do to one side of the equation you must do to the other.

I'll solve an example similar to yours: Solve for B in 0.9C - 10B = A.

First I subtract 0.9C from both sides of the equation:

0.9C - 10B - 0.9C = A - 0.9C

-10B = A - 0.9C

This "moves" the 0.9C from one side to the other, because it has cancelled with the positive 0.9C on the left side of the equation.

Now I have a factor of -10 in front of the B, so to move that out of there I need to cancel it away by dividing by -10. So I have to divide both entire sides by -10:

(-10B) / (-10) = (A - 0.9C) / -10

B = A/-10 - 0.9C/-10

B = -A/10 + 0.09C

B = 0.09C - A/10

So now I have solved for B.

Try your problem in a similar way, Ben.

Cheers,
Stephen La Rocque.     Math Central is supported by the University of Regina and The Pacific Institute for the Mathematical Sciences.