In this task, you need to answer the given multiple-choice question on the physics. Classify your answers into 'a', 'b', 'c', 'd', and 'e'.

Q: Problem: a man rows his boat 75 km downstream and 45 km upstream , taking 2 1 / 2 hours each time . find the speed of the stream ? 
Options: a ) 6 , b ) 7 , c ) 5 , d ) 8 , e ) 9

A: a
****
Q: Problem: a river boat leaves silver town and travels upstream to gold town at an average speed of 3 kilometers per hour . it returns by the same route at an average speed of 7 kilometers per hour . what is the average speed for the round - trip in kilometers per hour ? 
Options: a ) 4.2 , b ) 7.1 , c ) 7.2 , d ) 7.5 , e ) 8.0

A: a
****
Q: Problem: a work as fast as b . if b can complete a work in 20 days independently , the number of days in which a and b can together finish the work in ? 
Options: a ) 2 days , b ) 3 days , c ) 4 days , d ) 5 days , e ) 6 2 / 3 days

A:
e
****