QUESTION

CSAT

Medium

Maths

Prelims 2025

Consider the first 100 natural numbers. How many of them are not divisible by any one of 22, 33, 55, 77, and 99?

Select an option to attempt

Explanation

We need to find how many numbers from 1 to 100 are notnot divisible by 2, 3, 5, 7, or 9.

Notice that:

  • 100=10\sqrt{100} = 10, so any non-prime 100\le 100 must be divisible by a prime 10\le 10.
  • The primes 10\le 10 are 2, 3, 5, and 7.

Hence, any number in 1–100 that is notnot divisible by 2, 3, 5, or 7 must itself be a prime numberprime\ number greater than 7 (since smaller ones divide others).

Additionally, 9=329 = 3^2, so divisibility by 9 is already covered under 3.

So, we simply count:

  • Prime numbers between 1 and 100 = 25
  • Exclude 2, 3, 5, and 7 (since they are divisible by themselves): 254=2125 - 4 = 21
  • Include 1, because 1 is notnot divisible by any of these numbers.

Therefore, total = 21+1=2221 + 1 = 22

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