When Zara was 50 years old, her son was 3 times her daughter's age. Zara would be twice her son's age when her daughter was 35 years old. How old would Zara be when her daughter was 21 years old?

	Zara	Son	Daughter
Before	50	3 u	1 u
Change	+ ?	+ ?	+ ?
After	2 p	1 p	35

The difference in the ages between Zara and her son remains unchanged.

Difference in Zara's and her son's ages at first
50 - 3 u = 2 p - 1 p
50 - 3 u = 1 p --- (1)

The difference in the ages between her son and her daughter remains unchanged.

Difference in her son's and her daughter's ages at first
3 u - 1 u = 1 p - 35
2 u + 35 = 1 p --- (2)

Make p the same.

(1) = (2)
50 - 3 u = 2 u + 35
2 u + 3 u = 50 - 35
5 u = 15

1 u = 15 ÷ 5 = 3

Age difference between Zara and her daughter
= 50 - 1 u
= 50 - 3
= 47

Zara's age when her daughter was 21 years old
= 47 + 21
= 68

Answer(s): 68