Question

Find the number of 4-digit numbers that contain b at least three odd digits.

Answer 1

Answer:

  3000

Step-by-step explanation:

There are 5 odd digits, so 5^3 = 125 ways to have 3 odd digits.

Given 3 odd digits, the first digit can be even, but not zero, so there are 4 ways to do that.

The second, third, or fourth digits can be even 5 ways, so there are ...

  125(4 +5 +5 +5) = 2375

ways to have 3 odd digits in a 4-digit number.

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There are 5^4 = 625 ways to have 4 odd digits.

The number of 4-digit numbers with 3 or 4 odd digits is ...

  2375 +625 = 3000