Givens
Call one of the angles = x
The other one is 180 - x
They are in the ratio of 3 to 5
Equation
3/5 = x / (180 - x)
Solve
3/5 = x / (180 - x) Cross Multiply
3*(180 - x) = 5x
540 - 3x = 5x Add 3x to both sixes
540 = 5x + 3x
540 = 8x
Divide both sides by 8
540 / 8 = x
x = 67.5
One angle is 67.5
The supplement is 180 - 67.5 = 112.5
Check
67.5/112.5 = 0.6 which is the same thing as 3/5