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: if a - b = 5 and a 2 + b 2 = 31 , find the value of ab . 
Options: a ) 9 , b ) 2 , c ) 3 , d ) 7 , e ) 12
c

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: a certain store sells all maps at one price and all books at another price . on monday the store sold 12 maps and 10 books for a total of $ 38.00 , and on tuesday the store sold 12 maps and 9 books for a total of $ 30.00 . at this store , how much less does a map sell for than a book ? 
Options: a ) $ 0.25 , b ) $ 0.50 , c ) $ 0.75 , d ) $ 1.00 , e ) $ 4.5

e