Question 3936
Let the symbol D stand for Dorothy's current age and the symbol S stand for her sisters current age.

Dorothy is currently 3 times her sisters age. This can be written as:

D = 3 x S

In 5 years Dorothy's age will be D + 5 and her sisters age will be S + 5. At this time Dorothy is twice as old as her sister. Therefore:

(D + 5) = 2 x (S + 5)
 D + 5  = 2S + 10

Substitute the first equation into the equation above and you get:

3S + 5 = 2S + 10
S = 5

Substitute S = 5 into the first equation and you get D = 15. That means that Dorothy's current age is 15 and her sisters current age is 5.