Question

a man is 42yrs old and son is 12 yrs old in how many yrs will the age of son ne half the age of the man at that time

Answer 1

The age of men=42 Years

Age of son =12

Let x = the unknown number of years

The man is now 42 and his son is 12.

Man's Age = (42 + x) years old

Son's Age  = (12 + x) years old

Now in x years, the son will be one-half as old as his dad.  That's the key piece of information that allows us to equate their two ages:

Son's Age = (1/2)*Man's Age

12 + x      =   (1/2)*(42 + x)

12 + x = 21 + x/2