The cross-section of a water channel is a trapezium in shape. If the channel is 14 meters wide at the top and 8 meters wide at the bottom and the area of cross-section is 770 square meters, what is the depth of the channel (in meters)? Choose one
A) 50 B) 60 C) 70 D) 80 E) 90

Rationale: "1 / 2 * d * ( 14 + 8 ) = 770 d = 70 the answer is c ."
Answer: C