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Question

How many three digit whole numbers are there between 75 and 405?

This question was previously asked in
RRB ALP 2018 CBT 2 Fitter Question Paper (21-Jan-2019) (Shift 3)
The correct answer is

305

Understanding Three-Digit Whole Numbers

The question asks us to find the count of three-digit whole numbers that fall between 75 and 405. First, let's clarify what three-digit whole numbers are and what it means to be "between" two numbers.

  • Whole Numbers: These are non-negative integers (0, 1, 2, 3, ...).
  • Three-Digit Numbers: These are whole numbers that have exactly three digits. The smallest three-digit number is 100, and the largest is 999.

So, the set of all three-digit whole numbers is {100, 101, 102, ..., 998, 999}.

Finding Numbers Between 75 and 405

The phrase "between 75 and 405" means the numbers must be strictly greater than 75 and strictly less than 405. We are looking for three-digit whole numbers that satisfy this condition.

  • Is 75 a three-digit number? No, it has two digits.
  • Is 405 a three-digit number? Yes, but the numbers must be *less than* 405.

We need to find the three-digit numbers that are greater than 75 and less than 405.

Identifying the Range of Numbers

The three-digit numbers start from 100. Since 100 is greater than 75, our range starts at 100.

The numbers must be less than 405. The three-digit numbers just below 405 are 404, 403, and so on. The number 404 is a three-digit number and is less than 405. The number 405 is not included because the range is strictly *between* 75 and 405.

So, the three-digit whole numbers we are interested in are those from 100 up to 404, inclusive. This sequence is 100, 101, 102, ..., 403, 404.

Counting the Numbers in the Range

To count the number of whole numbers in a continuous range from a starting number (first) to an ending number (last), we can use the formula:

Number of terms = Last number - First number + 1

In our case:

  • First number = 100
  • Last number = 404

Using the formula, the count is:

\( \text{Count} = 404 - 100 + 1 \)

\( \text{Count} = 304 + 1 \)

\( \text{Count} = 305 \)

Therefore, there are 305 three-digit whole numbers between 75 and 405.

Final Answer Calculation

The three-digit whole numbers between 75 and 405 are the numbers in the set {100, 101, ..., 404}.

To find the count, we subtract the smallest number from the largest number in the set and add 1.

\( \text{Total Count} = 404 - 100 + 1 = 305 \)

The number of such numbers is 305.

Revision Table: Key Concepts

Concept Description Example
Whole Numbers Non-negative integers (0, 1, 2, ...) 0, 10, 150, 400
Three-Digit Number A whole number with exactly three digits 100, 567, 999
Numbers Between A and B Numbers > A and < B (A and B are not included) Numbers between 5 and 10 are 6, 7, 8, 9
Counting Numbers in a Range (inclusive) Last number - First number + 1 Count of numbers from 10 to 15 is 15 - 10 + 1 = 6 (10, 11, 12, 13, 14, 15)

Additional Information: Number Systems

Understanding different types of numbers is fundamental in mathematics.

  • Natural Numbers: These are the positive integers starting from 1 (1, 2, 3, ...). Also called counting numbers.
  • Whole Numbers: Natural numbers including zero (0, 1, 2, 3, ...).
  • Integers: Whole numbers and their negative counterparts (...-3, -2, -1, 0, 1, 2, 3...).
  • Rational Numbers: Numbers that can be expressed as a fraction \( \frac{p}{q} \), where \( p \) and \( q \) are integers and \( q \neq 0 \).
  • Irrational Numbers: Numbers that cannot be expressed as a simple fraction, like \( \sqrt{2} \) or \( \pi \).
  • Real Numbers: All rational and irrational numbers.

In this problem, we specifically dealt with whole numbers that meet a digit count and range criteria.

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