Answer:
The mother is 36 years now.
Explanation:
x - the age of the daugther now
3x - the age of the mother now
x + 12 - the age of the daugther in 12 years' time
3x + 12 - the age of the mother in 12 years' time
The mother will be twice as old as her daughter will be then:
3x + 12 = 2·(x + 12)
3x + 12 = 2x + 24 {subtract 2x from both sides}
x + 12 = 24 {subtract 12 from both sides}
x = 12 (the age of the daughter now)
3x = 3·12 = 36