The cross-section of a tunnel is a trapezium in shape. If the tunnel is 15 m wide at the top and 5 m wide at the bottom and the area of cross-section is 400 sq m, the depth of tunnel is?
Choose the correct choice from the following:
A) 200 m
B) 600 m
C) 400 m
D) 800 m
E) 900 m

Rationale: 1 / 2 * d ( 15 + 5 ) = 400 d = 400 answer : c
Answer: C