The cross-section of a river bank is a trapezium in shape. If the river bank is 12 m wide at the top and 8 m wide at the bottom and the area of cross-section is 500 sq m, the depth of river bank is?
Choose the correct answer: A) 20 m B) 60 m C) 40 m D) 50 m E) 90 m

Rationale: 1 / 2 * d ( 12 + 8 ) = 500 d = 50 answer : d
Answer: D