How did you know the measure of abc was 90

Question

It never states in the question that the whole angle is 90

Answer 1

I didn’t answer the original question, but my guess is that the person who answered made an assumption so that a solution could be given. The only other simple angle would be if ABC was a straight line so angle ABC would have been 180, making 8x=170, and x=21.25. But that doesn’t look cool, so they went for ABC=90 instead, and x=10 looks like a cool answer.