a + b = 8
a + c = 5
Subtracting the second equation from the first:
b - c = 3
b + c = 3
Adding the two equations together:
2b = 6
b = 3
Since a + b = 8 and b = 3: a + 3 = 8 ---> a = 5
Since a + c = 5 and a = 5: c = 0
From these values, you can get the answer.
