UNIT DIGITS

Number Theory Level pending

What will be the unit digit of 1!+2!+3!+......+99!

0 3 5 9

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1 solution

Jackson Abascal
Nov 15, 2014

From 5! and up, the number will be divisible by 10 and therefore end in a 0, since 5! contains 2 x 5. All we need to do is then add the first 4 factorials, 1+2+6+24=33 and take the units digit, 3.

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