When Min was 49 years old, her daughter was 4 times her son's age. Min would be twice her daughter's age when her son was 21 years old. How old would Min be when her son was 3 years old?

	Min	Daughter	Son
Before	49	4 u	1 u
Change	+ ?	+ ?	+ ?
After	2 p	1 p	21

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

Difference in Min's and her daughter's ages at first
49 - 4 u = 2 p - 1 p
49 - 4 u = 1 p --- (1)

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

Difference in her daughter's and her son's ages at first
4 u - 1 u = 1 p - 21
3 u + 21 = 1 p --- (2)

Make p the same.

(1) = (2)
49 - 4 u = 3 u + 21
3 u + 4 u = 49 - 21
7 u = 28

1 u = 28 ÷ 7 = 4

Age difference between Min and her son
= 49 - 1 u
= 49 - 4
= 45

Min's age when her son was 3 years old
= 45 + 3
= 48

Answer(s): 48