Take 87-63 = 24, which is the difference of the 2 hours. Divide this by two to get the hourly rate, which is $12.
We can now set up the equation as follows:
4(12) + x = 63
6(12) + x = 87
Now solve for each:
4(12) + x =63
48 + x = 63
x = $15
6(12) + x = $87
72 + x = 87
x = $15
Therefore, the Flat Fee = $15 and the Hourly rate = $12