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Colossal Sum

Age 11 to 14 Short
Challenge Level

Answer: $0$

$9^1=9$
$9^2=81$ units digit $1$
$9^3=9\times81$
        units digit comes from $9\times1=9$
$9^4=9\times9^3$
        units digit comes from $9\times9$ so it will be $1$

And so on - units digits go $9,1,9,1,9,1...$

So the last digit of this sum will come from $9+1=10$ - so it will be $0.$


You can find more short problems, arranged by curriculum topic, in our short problems collection.