Suppose you know that ∠S and ∠Y are complementary, and that m∠S = 2 (m∠Y) - 45°. Find m∠Y.

If S and Y are complementary, that means they add to 90.

So S + Y = 90

And S = 2Y - 45

Use substitution to plug 2Y - 45 in for S.

(2Y - 45) + Y = 90

3Y - 45 = 90

3Y = 135

Y = 45

If Y equals 45, then S also equals 45 since S+Y=90.