The speed of a boat in still water is 20 km/hr and the rate of current is 5 km/hr. The distance travelled downstream in 21 minutes is:
Choose the correct answer:
A) 9.75
B) 5.75
C) 8.75
D) 6.75
E) 5.15

Rationale: "explanation : speed downstream = ( 20 + 5 ) kmph = 25 kmph distance travelled = ( 25 * ( 21 / 60 ) ) km = 8.75 km . answer : c"
Answer: C