Question 636714
Maria is 18 years old now
Let x=Anna's age now
Let y=number of years that have passed since Maria was x years old
Sooo
18-y=x
x+y=18---eq1
Then when Maria was x years old , Anna was x-y years old
Sooo
18=2(x-y)=2x-2y or
x-y=9----eq2

Add eq1 and eq2 and we get:
2x=27
x=13.5 Anna's age now
substitute 13.5 into eq1
y=18-13.5=4.5 number of years that have passed since Maria was 13.5 years old. So, when Maria was 13.5, Anna was 13.5-4.5=9
18=2*9

Hope this helps---ptaylor