Given triangle DEF where ∠D = (7b +1)∘, ∠E = (2b + 1)∘, and ∠F = ( b + 8)∘. What is the measure of ∠E?

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Explanation:
For any triangle, the three interior angles always add to 180.
D+E+F = 180
(7b+1) + (2b+1) + (b+8) = 180
(7b+2b+b) + (1+1+8) = 180
10b+10 = 180
10b = 180-10
10b = 170
b = 170/10
b = 17
Use this value of b to find the three angle measures
As a check,
D+E+F = 120+35+25 = 120+60 = 180
which helps confirm the correct answers.