let s is Marys sister age and m is Marys age. According the presentation we have.
s = m + 7 (1) and s + m = 35 (2). from the (2) we get m = 35 - s plugging this value of m to (1) we get
s = 35-s+7 --->>> 2s=42 so s=42/2 = 21 and from m=35-s we get m=35-21 so m = 14
so marys age is 14 years and marys sister age is 21 years