In this problem we are given that the measures of the angles of a triangle are in the ratio 1:4:5 and we are asked to find the measure of each angle. Remember that when the ratio of the angles is 1:4:5, we can represent the angles as 1x, 4x, and 5x. So let's call 1x or just X our first angle, 4x our second angle, and 5x our third angle. Now since the measures of the angles of a triangle add to 180 degrees we can set up the equation shown below
x + 4 x + 5 x = 180
Simplifying on the left side we have 10x = 180 and dividing both sides by 10, x = 18. Now to find the measures of each of our angles, we plug an 18 back in for x. So our first angle X measures 18 degrees, our second angle measures 4 x 18 or 72 degrees, and our third angle 5x measures 5 x 18 or 90 degrees.