Given the image attached, where the altitude 30 is the one in red, 20 the one in green.
CE / AD = BC / AB
20 / 30 = 2 / 3
BC = 2 / 3 AB
BD = 1 / 2 BC = 1 / 3 AB
Using Pythgorean theorem
BD^2 + AD^2 = AB^2
(1/3AB)^2 + 30^2 = AB^2
(1/9AB)^2 + 900 = AB^2
AB^2 + 8100 = 9AB^2
8AB^2 = 8100
AB^2 = (8100/8)
AB = sqrt(8100/8)
AB = 45 / sqrt(2)
Looking for the base angle
sin B = AD/AB
sin B = 30 / [45/sqrt(2)]
sin B = 0.9426090416
B = sin^-1 (0.9426090416)
B = 70.53 degrees
