What is the unit digit of the sum of first 111 whole numbers

Options

A. 4
B. 5
C. 6
D. 0

Answer

To find the unit digit of the sum of the first 111 whole numbers, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Whole Numbers**:
The first 111 whole numbers are 0, 1, 2, 3, ..., 110.

2. **Count the Numbers**:
Since we start counting from 0, the total count of whole numbers from 0 to 110 is indeed 111.

3. **Use the Formula for the Sum of Whole Numbers**:
The formula for the sum of the first n natural numbers is given by:
\[ S = \frac{n(n + 1)}{2} \]
However, since we are considering whole numbers starting from 0, we will use the formula for the sum of the first 110 natural numbers (which is the same as the first 111 whole numbers).

4. **Substitute n = 110 into the Formula**:
\[ S = \frac{110 \times (110 + 1)}{2} = \frac{110 \times 111}{2} \]

5. **Calculate the Product**:
First, calculate \(110 \times 111\):
\[ 110 \times 111 = 12210 \]

6. **Divide by 2**:
Now, divide 12210 by 2:
\[ \frac{12210}{2} = 6105 \]

7. **Find the Unit Digit**:
The unit digit of 6105 is 5.

### Final Answer:
The unit digit of the sum of the first 111 whole numbers is **5**.

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