SOLUTION: The sum of the ages of Maria and Anna is 35. When Maria was two thirds her present age and Anna was 3/4 of her present age, the sum of their ages was 25. How old is Maria now?

M     +   A  = 35,       (1)

 +  = 25       (2)


Multiply equation (2) by 12 (both sides).  Keep equation (2) as is.


 M +  A = 35             (1')

8M + 9A = 300            (2')


From (1), express  A = 35 - M and substitute into (2'). You will get


8M + 9*(35-M) = 300,


M =  = 15.


ANSWER.  Maria is 15 years old.