\(\text{Do you actually mean }(30 + A) + (10A + B)= 10B\;\text{?}\)
If so we will have to try each positive 1-digit integers.
First substitute A = 1
31 + 1B = B0
B have to be 9 so A = 1 didn't work.
Substitute A = 2
32 + 2B = B0
B have to be 8, so A = 2 didn't work.
Substitute A = 3
33 + 3B = B0
B have to be 7. We did it.
33 + 37 = 70.
B = 7