Calculate the sum if two different conditions

Hello,
If in column a:a if have numbers 1 to 5 and in column b:b i have a and b randomly and finally in cloumn c:c, i have different time say, 12min, 20 min etc, then In sheet two I want the sum of all the time if in cell a1 it is 2 and cell b1 it is a and like wise. Basically if
A B C
1 b 12
2 a 10
2 a 05
4 b 04
5 b 15

what i want is a formula that will calculate 10 and 05 and give the result as 15