ABCD is a square and BCE is an equilateral triangle drawn inside it on side BC. Find ∠AED.

BE = BC = AB and ∠ABE = 90° − 60° = 30°, so in isosceles triangle ABE, ∠BAE = 75° and ∠EAD = 15°. By symmetry ∠EDA = 15°, so ∠AED = $180°-30°=150°$.