As you might know, 3 angles of a triangle adds up to 180*. Since you already know the angles are 60* there is one more 60*. So by this you know that all the sides are equal.
As the given states, one side is 20. So following the standard way to get the area of a triangle b*h/2; 20*20 = 400 / 2 = 200 sq units
