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'.

Q: Problem: of the final grades received by the students in a certain math course , 1 / 5 are a ' s , 1 / 4 are b ' s , 1 / 2 are c ' s , and the remaining 30 grades are d ' s . what is the number of students in the course ? 
Options: a ) 80 , b ) 110 , c ) 160 , d ) 600 , e ) 400

A: d
****
Q: Problem: a seller of used cars has 16 cars to sell and each of his clients selected 2 cars that he liked most . if each car was selected exactly thrice , how many clients visited the garage ? 
Options: a ) 8 , b ) 10 , c ) 12 , d ) 14 , e ) 24

A: e
****
Q: Problem: cost is expressed by the formula tb ^ 4 . if b is doubled , the new cost q is what percent of the original cost ? 
Options: a ) q = 200 , b ) q = 600 , c ) q = 800 , d ) q = 1600 , e ) q = 50

A:
d
****