Let A, B and C be the three angles in AP (in degrees)

And A being the least and C being the greatest

Then

C = 2A [ Given][ eqn i]

B - A = C - B

2B = A + C

2B = A + 2A [ using eqn i]

2B = 3A

B = [ eqn ii]

Using the angle sum property of triangle

A + B + C = 180^O

[ using i and ii]

9A = 360

A = 40^O

[ using ii]

C = 2(40) = 80^O [ using i]