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JEE Main 2023
Permutations & Combinations
Permutations and Combinations
Easy

Question

If the digits are not allowed to repeat in any number formed by using the digits 0, 2, 4, 6, 8, then the number of all numbers greater than 10,000 is equal to _____________.

Answer: 4

Solution

Key Concepts and Formulas

  • Permutation: The number of ways to arrange r objects from a set of n distinct objects, denoted by P(n,r)P(n, r) or nPr_nP_r, is given by P(n,r)=n!(nr)!P(n, r) = \frac{n!}{(n-r)!}.
  • Counting Principle: If there are m ways to do one thing and n ways to do another, then there are m × n ways to do both.
  • Number greater than 10,000: For a number to be greater than 10,000, it must be a 5-digit number since we are only using the digits 0, 2, 4, 6, and 8.

Step-by-Step Solution

Step 1: Determine the possible digits for each place value.

Since we want to form numbers greater than 10,000 using the digits 0, 2, 4, 6, 8 without repetition, we need to form 5-digit numbers. Let's consider the possible choices for each digit place: ten-thousands, thousands, hundreds, tens, and units.

Step 2: Count the number of choices for the ten-thousands place.

The ten-thousands place cannot be 0, otherwise the number would not be a 5-digit number. Therefore, we have 4 choices (2, 4, 6, or 8) for the ten-thousands place.

Step 3: Count the number of choices for the thousands place.

After choosing a digit for the ten-thousands place, we have 4 remaining digits to choose from for the thousands place (including 0).

Step 4: Count the number of choices for the hundreds place.

After choosing digits for the ten-thousands and thousands places, we have 3 remaining digits to choose from for the hundreds place.

Step 5: Count the number of choices for the tens place.

After choosing digits for the ten-thousands, thousands, and hundreds places, we have 2 remaining digits to choose from for the tens place.

Step 6: Count the number of choices for the units place.

After choosing digits for the ten-thousands, thousands, hundreds, and tens places, we have only 1 remaining digit to choose from for the units place.

Step 7: Apply the counting principle to find the total number of such numbers.

The total number of 5-digit numbers greater than 10,000 that can be formed using the digits 0, 2, 4, 6, 8 without repetition is:

4×4×3×2×1=964 \times 4 \times 3 \times 2 \times 1 = 96

Common Mistakes & Tips

  • Remember that the first digit (ten-thousands place) cannot be zero. This is a common mistake.
  • Be careful not to repeat digits. The problem specifically states that digits are not allowed to repeat.
  • The counting principle is crucial for solving permutation and combination problems. Make sure you understand how to apply it.

Summary

We are asked to find the number of 5-digit numbers greater than 10,000 that can be formed using the digits 0, 2, 4, 6, 8 without repetition. The ten-thousands place has 4 choices (2, 4, 6, or 8). Then the thousands place has 4 choices, the hundreds place has 3 choices, the tens place has 2 choices, and the units place has 1 choice. Multiplying these choices together gives us 4×4×3×2×1=964 \times 4 \times 3 \times 2 \times 1 = 96. The question is asking for the last two digits of this number, so the answer is 96, thus the last two digits are 96.

The final answer is \boxed{96}.

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