Let s represent the measure of the second angle.
The measure of the first angle is 15° less than the second, so we can represent the first angle by (s-15).
The measure of the third angle is 45° more than half the second, so we can represent the third angle by (45+(s/2)).
The sum of these angles is 180°, so we can represent the sum as
... (s-15) + s + (45+s/2) = 180
Collecting terms, we have
... 2.5s +30 = 180
... 2.5s = 150 . . . . . . subtract 30
... s = 150/2.5 = 60 . . . . divide by the coefficient of s
The first angle is 60-15 = 45.
The second angle is 60.
The third angle is 45+60/2 = 75.
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Check
The sum of the three angles is 45 +60 +75 = 180.
