Tom bought a candy bar and gave the clerk a quarter, a dime, and a nickel and received one penny in change. how much did the candy bar cost?

A quarter is worth $0.25
A dime is worth $0.10
and a nickel is worth $0.05
He got $0.01 back

to see how much he paid for we add up the money
so 
0.25 + 0.10 + 0.05 = 0.40
Then he received one penny back so
0.40 - 0.01 = 0.39
The candy bar cost $0.39 or 39¢

Hope this helps :)