what is the unit digit of the sum of first 111 whole numbers?.

June 28, 2026

The unit digit of the sum of the first 111 whole numbers is 5.

Quick Scoop: Final Answer

We are summing the first 111 whole numbers:
0+1+2+3+⋯+1100+1+2+3+\dots +1100+1+2+3+⋯+110. This is the same as the sum of the first 110 natural numbers, since adding 0 does nothing to the sum. The sum of the first nnn natural numbers is:

S=n(n+1)2S=\frac{n(n+1)}{2}S=2n(n+1)

Here n=110n=110n=110, so:

S=110×1112=55×111S=\frac{110\times 111}{2}=55\times 111S=2110×111=55×111

To get the unit digit , we only care about the unit digits of the factors:

Unit digit of 55 → 5
Unit digit of 111 → 1

Now multiply unit digits:

5×1=55\times 1=55×1=5

So, the unit digit of the sum is 5.