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'.

[EX Q]: Problem: 1 , 6 , 16 , 31 , 51 , ( . . . . ) 
Options: a ) 61 , b ) 66 , c ) 76 , d ) 86 , e ) 83
[EX A]: b

[EX Q]: Problem: company x provides bottled water to its employees in 5 liter bottles , $ 2 each . at least how many 5 l bottles must company x buy monthly , so that new contract with $ 100 fixed monthly rate and $ 1 for 10 l bottle each paid off ? ( assume that no other costs apply ) 
Options: a ) 121 , b ) 125 , c ) 132 , d ) 134 , e ) 138
[EX A]: d

[EX Q]: Problem: if a : b = 3 : 4 , b : c = 7 : 9 , c : d = 5 : 7 , find a : d ? 
Options: a ) 5 / 12 , b ) 5 / 15 , c ) 5 / 10 , d ) 5 / 11 , e ) 5 / 13
[EX A]:
a