Answer: $40
Explanation:
If the worth of the stamp is double that of its original price 3 years ago ($1.25), the current price of the stamp should be $2.50. If that doubles every 3 years the answer should look something like this:
3 years: $5 (2.50X2=5)
6 years: $10 (5X2=10)
9 years: $20 (10X2=20)
12 years $40 (20X2=40)
In 12 years the stamp will be worth $40.