In △ ABC, ∠ a - ∠ B = ∠ B - ∠ C = 15 °, find the degree of ∠ a ∠ B ∠ C

In △ ABC, ∠ a - ∠ B = ∠ B - ∠ C = 15 °, find the degree of ∠ a ∠ B ∠ C

A-B=15(1)
A+B+C=180(2)
B-C=15(3)
There are three unknowns in the above three formulas, so just solve the equation