the angles in an equilateral triangle are all 60 degrees
and the altitude will be obtained from the equation
tan 60 = a / 10 where a = altitude and 1/2 base of triangle = 1/2 * 20 = 10
a = 10 tan 60 = 10 sqrt3
so the area = 10* 10 sqrt3 = 100 sqrt3 in^2
or 173 in^2 to neraest in^2
