In ΔABC, if ∠A – ∠B = 42° and ∠B – ∠C = 21°, then ∠B = ?
A = B + 42
C = B – 21
A + B + C = 180
B + 42 + B + B – 21 =180
3B + 21 = 180
3B = 159
B =