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: exactly 3 / 5 of the people in the room are under the age of 21 , and exactly 5 / 13 of the people in the room are over the age of 65 . if the total number of the people in the room is greater than 50 and less than 100 , how many people in the room are under the age of 21 ? 
Options: a ) 21 , b ) 35 , c ) 39 , d ) 60 , e ) 42
e

Problem: if 3 x – 5 y = 5 and x / x + y = 5 / 7 , then what is the value of x – y ? 
Options: a ) 9 , b ) 6 , c ) 4 , d ) 3 , e ) none of these
d

Problem: what is the average of odd numbers from 1 to 79 ? 
Options: a ) 30 , b ) 25 , c ) 35 , d ) 40 , e ) 45

d