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: a man is 20 years older than his son . in two years , his age will be twice the age of his son . the present age of the son is ?
Options: a ) 11 , b ) 25 , c ) 27 , d ) 18 , e ) 91
[A]: d

[Q]: Problem: the unit digit in the product ( 784 x 618 x 917 x 463 ) is :
Options: a ) 5 , b ) 9 , c ) 16 , d ) 2 , e ) 42
[A]: d

[Q]: Problem: village x has a population of 68000 , which is decreasing at the rate of 1200 per year . village y has a population of 42000 , which is increasing at the rate of 800 per year . in how many years will the population of the two villages be equal ?
Options: a ) 15 , b ) 19 , c ) 11 , d ) 18 , e ) 13
[A]:
e