Definition of supplementary angles:
Two Angles are Supplementary when they add up to 180 degrees. Notice that together they make a straight angle.
m∠A +m∠B =180°............(1)
We are given :
m∠A is five times the sum of the m∠B plus 7.2°
m∠A = 5(m∠B +7.2) .............. (2)
Plugging value of angle A from equation (2) in (1),
5(m∠B +7.2) + m∠B =180
5m∠B +36 + m∠B =180
6m∠B =180-36
m∠B = 24°
So angle B is 24 degrees.
