Tennnnnn's digit
Number Theory
Level
pending
Find the ten's digit of the expression:
(1! + 2! + 3! + ... + 100!)
cannot be determined, only unit digit can be determined
0
5
1
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1 solution
The ten's digit is only dependent on (1! +2! + 3! + ... + 9! ) as 10! has two zeros and the number of trailing zeros keeps increasing thereafter. So, 1 + 2 + 6 + 24 + 20 + 20 + 40 + 20 + 80 (only last two digits) = 1(ten's digit)