The sum of the ages of two sisters is 35. the younger sister is 3 more than 1/3 of the older sister's age. how old is the younger sister?

Set the sisters' ages to x and y 
x + y = 35

let's say x = younger sister's age
and y = older sister's age

so
x = [(1/3)*y] + 3

plug in the value above for x in the equation x + y = 35

solve for y, then for x

remember which variable you set for the younger sister, and make sure that you use that variable as your answer

Hope this helped!