A boat goes 100 km downstream in 4 hours, and 75 Km upstream in 15 hours. The speed of the stream is?  Choose the correct choice from the following choices
A) 10
B) 20
C) 25
D) 35
E) 15

Rationale: "100 - - - 10 ds = 25 ? - - - - 1 75 - - - - 15 us = 5 ? - - - - - 1 s = ( 25 - 5 ) / 2 = 10 kmph . answer : a"
Answer: A