A boat moves upstream at the rate of 1 km in 40 minutes and down stream 1 km in 12 minutes. Then the speed of the current is :  Pick one:
A) 1 kmph  B) 2 kmph  C) 1.75 kmph  D) 2.5 kmph  E) 3.5 kmph

Rationale: "rate upstream = ( 1 / 40 * 60 ) = 1.5 kmph rate dowm stream = 1 / 12 * 60 = 5 kmph rate of the current = ½ ( 5 - 1.5 ) = 1.75 kmph answer : c"
Answer: C