Answer:
Let's define the variables:
M = mother's age
D = daughter's age.
"a mother is twice as old as her daughter"
This can be written as:
M = 2*D
"5 years from now she will be thrice as old as her daughter"
(M + 5) = 3*D
We want to know how old is the mother five years from now.
First, our equations are:
M = 2*D
(M + 5) = 3*D
First, we need to isolate one of the variables in one of the equations, because for the solution we want the mother's age, we should isolate the daughter's age.
Let's isolate D in the first equation:
M/2 = D
Now we can replace this in the other equation:
Now we can replace this on the other equation:
(M + 5) = 3*(M/2)
M + 5 - (3/2)*M = 0
-(1/2)*M + 5 = 0
5 = (1/2)*M
2*5 = M = 10
So the mother is 10 years old (this hasn't a lot of sense, maybe there is a wrong number on the question)
and 5 years from now, the mother will be:
10 + 5 = 15 years old.