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

Problem: in a river flowing at 2 km / hr , a boat travels 40 km upstream and then returns downstream to the starting point . if its speed in still water be 6 km / hr , find the total journey time . 
Options: a ) 10 hours , b ) 12 hours , c ) 14 hours , d ) 15 hours , e ) none of these
d

Problem: the average of 11 numbers is 60 . out of 11 numbers the average of first 6 no . is 78 , and last 6 numbers is 75 then find 6 th number ? 
Options: a ) a ) 258 , b ) b ) 65 , c ) c ) 58 , d ) d ) 62 , e ) e ) 48
a

Problem: what is the greatest prime factor of 3 ^ 8 - 1 ? 
Options: a ) 29 , b ) 31 , c ) 37 , d ) 41 , e ) 43

d