Question 1190940
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Edit: I changed the 40 to 36. I'm not sure why my brain went for that error initially.


Difference between their ages is a perfect square number. 
The difference in their age is between 5 years and 40 years.


Those two facts allow us to say that their age gap is one of the following numbers: 9, 16, 25, 36
Simply list the perfect squares between 5 and 40.
Later on, we'll only focus on the smallest and largest age gap. 


x = sister's present day age
2x = Linda's present day age


If Linda is 9 years older, then
(linda's age) - (sister's age) = 9
2x - x = 9
x = 9
Telling us that the sister is 9 years old and Linda is 2*x = 2*9 = 18 years old.


Or if Linda is 36 years older, then you should find that x = 36 through similar steps. That would make Linda to be 2x = 2*36 = 72 years old.


Linda's lowest age = 18
Linda's highest age = 80
Difference = 72 - 18 = 54


Answer: 54
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