Answer:
12 years
Explanation:
Represent the mother's age by m and the son's by 4. Currently m = 36 years old.
In x years the mother will be three times as old as her son:
mother: 36 + x years old
son: 4 + x years old
The pertinent equation to solve is:
36 + x = 3(4 + x)
Performing the multiplication, we get:
36 + x = 12 + 3x, or
24 = 2x. Then x = 12.
Check: In 12 years, will the mother's age be 3 times the son's age?
Does 36 + 12 = 3(4 + 12)?
Does 48 = 48? YES
The mother will be 3 times as old as her son in 12 years from now.