What is the unit digit of the sum of the first 111 whole numbers, starting from 0 and going up to 111?

Introduction / Context:
This question involves the formula for the sum of the first n natural or whole numbers and then focuses on finding only the unit digit of that sum. It is a common style of quick arithmetic problem found in aptitude tests.

Given Data / Assumptions:
- We consider whole numbers from 0 to 111 inclusive.
- The sum can be calculated using the standard formula for the sum of the first n natural numbers.
- We only need the unit digit of the resulting sum, not the full value.

Concept / Approach:
The sum S of the first n natural numbers from 1 to n is given by S = n * (n + 1) / 2. Including 0 does not change the sum. After calculating S, we determine its unit digit by focusing on the unit digits of intermediate products, which is simpler than computing the entire number.

Step-by-Step Solution:
Step 1: Here n = 111 because we sum from 0 to 111.Step 2: Use the formula S = n * (n + 1) / 2.Step 3: Compute n + 1 = 111 + 1 = 112.Step 4: Form the product n * (n + 1) = 111 * 112.Step 5: First divide by 2 to simplify: 112 / 2 = 56, so S = 111 * 56.Step 6: Find the unit digit of 111 * 56 by using only unit digits: unit digit of 111 is 1, of 56 is 6, and 1 * 6 has unit digit 6.Step 7: Therefore the unit digit of the sum S is 6.

Verification / Alternative check:
We can compute the full product for confirmation. 111 * 56 equals 6216. The sum of the first 111 whole numbers is therefore 6216, and its unit digit is 6. This matches the result obtained from looking at unit digits only.

Why Other Options Are Wrong:
Option 4 might arise from miscalculating the product or from adding 110 numbers instead of 111. Option 5 comes from using incorrect formula variants. Option 0 or 2 may result from confusion about the pattern of unit digits for sums, but they do not match the actual computed sum. Only 6 corresponds to the correct unit digit.

Common Pitfalls:
Some learners mistakenly start at 1 instead of 0, but in this case it does not change the sum because 0 adds nothing. Others may try to manually add the numbers and introduce addition errors. Misreading the formula or incorrectly handling large products often causes wrong unit digits.

Final Answer:
The unit digit of the sum of the first 111 whole numbers is 6.