Use the cos ratio to figure this out. You have the length of the hypotenuse and the length of the leg on the ground. The hypotenuse is 20 and the leg is 10. The cos ratio says that cosA = side adjacent/hypotenuse. So fill in the blanks here: cosA = 10/20...cosA=1/2. In your calculator, you will enter inverse cos(.5) to find that the angle in question is 60 degrees. To find the inverse function on your calculator, press the "2nd" button and the "cos" then in the (), enter ".5".