Three pipes, A,B, and C, are often used two at a time to fill a tank. When A and B are used together, the tank is filled in 5 minutes. When either A and C or B and C are used together, the tank is filled in 3 minutes. How long would it take each pipe alone to fill the tank?

A + B = 5

A+ C = 3  so A = 3 -C

B + C = 3 so B = 3 - C

So 

3-C + 3 - C = 5

6 -2C =5

-2C = 5 -6

-2C = -1

C =0.5 minutes

A + C = 3 

A + 0.5 =3

A = 3 - 0.5 = 2.5 minutes

B = 3- 0.5 = 2.5 minutes