Question : What is the unit digit of the sum of the first 111 whole numbers?
Option 1: 4
Option 2: 6
Option 3: 5
Option 4: 0
Correct Answer: 5
Solution :
Given: The unit digit of the sum of the first 111 whole numbers.
The sum of first 111 whole numbers = 0 + 1 + 2 + 3 +.........+ 109 + 110
Using formula of sum of A.P. = $\frac{n}{2}[a+l]$
Where $a$ is the first term, $l$ is the last term of the A.P., and n is the number of terms.
= $\frac{111}{2}(0+110)$
= $55 × 111$
$\therefore$ Unit digit = 5 × 1 = 5
The unit digit of the sum of the first
111 whole numbers is 5.
Hence, the correct answer is 5.
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