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Powers of Four

Age 14 to 16 Short
Challenge Level

Answer: 15

$4^x+4^x+4^x+4^x = 4 \times 4^x$ = $4^{x+1}$ 

So $x+1=16$, hence $x = 15$

This problem is taken from the UKMT Mathematical Challenges.
You can find more short problems, arranged by curriculum topic, in our short problems collection.