Question

A father is 30 years older than his son. In 12 years, the man will be three times as old as his son. Find their present ages.

Answer 1

Let 
Present age of father = f
Present age of son = s
father is 30 years older than his son means if we add 30 years to the son's age father and so will be of same age
f=s+30............(1)
In 12 years, means we have add 12 in present ages of both
f+12=s+12
then 
the father will be twice as old as his son
f+12=2(s+12)
f=2s+24
f=2s+24-12
f=2s+12 .......(2) 
Put the value of f from (1) to (2)
s+30=2s+12
s=2s+12-30
s=2s-18
s-2s=-18
-s=-18
Multiply by (-1) both sides of the above equation
s=18 
Put the value of s in (1)
f=s+30
f=18+30
f=48 
Present age of father = 48
Present age of son = 18 

Answer 2

Let
Present age of father = x
Present age of son = y
father is 30 years older than his son means if we add 30 years to the son's age father and so will be of same age
x=y+30............(1)
In 12 years, means we have add 12 in present ages of both
x+12=y+12
then
the father will be twice as old as his son
x+12=2(y+12)
x=2y+24
x=2y+24-12
x=2y+12 .......(2)
Put the value of f from (1) to (2)
y+30=2y+12
y=2y+12-30
y=2y-18
y-2y=-18
-y=-18
Multiply by (-1) both sides of the above equation
y=18
Put the value of y in (1)
x=y+30
x=18+30
x=48
Present age of father = 48
Present age of son = 18