Answer:
19
Step by step :
If we say Justine is x years old than her uncle Stewart would be x + 13 years old
Stewart is 18 years older than Hana so Hana's age is x + 13 - 18 = x -5
If their mother is 8 years older than Stewart than she is x + 13 + 8 = x + 21 years old and this is 28 more than 12
Then we can say the mother is 40 and x + 21 = 40 ➡ x = 19 we said Justine is x years old therefore her age is 19
