The cross-section of a stream is a trapezium in shape. If the stream is 10 m wide at the top and the area of cross-section is 640 sq m, the depth of stream is 80m and width at the bottom is? Choose the correct choice from the following:
A) 2 m
B) 6 m
C) 4 m
D) 8 m
E) 9 m

Rationale: 1 / 2 * 80 ( 10 + b ) = 640 b = 6 m answer : b
Answer: B