What is the unit digit of the sum of first 111 whole numbers?
5
The first 111 whole numbers are \(0, 1, 2, \dots, 110\). Since adding 0 does not change the sum, this equals the sum of the first 110 natural numbers.
\[S=\frac{n(n+1)}{2}=\frac{110\times 111}{2}=55\times 111=6105\]
The unit digit of \(55\times 111\) is the unit digit of \(5\times 1=5\). Therefore the unit digit is 5.
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