Answer:
Mary - 14
Mary's sister - 21
Step-by-step explanation:
Let x be the age of Mary = x
age of Mary's sister = x + 7
Add up to 35
x + x + 7 = 35
2x = 35 - 7
2x = 28
2x/2 = 28/2
x = 14
Now substitute the value of x
Mary = 14
Mary's sister = 14 + 7 = 21
21 + 14 = 35
