Carol H. answered 09/07/18
Tutor
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Masters' Degree in Mathematics with 35 years Teaching Experience.
Your wording is slightly confusing. I think it should read ∠A and ∠B are supplementary. ∠B is three times the size of ∠A.
Let x = the measure of ∠A
Let 3x = the measure of ∠B
x + 3x = 180
4x = 180
x = 45º
3x = 135º