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Permutations and Combinations question

2025 · 24 Jan · Shift 1 · Q50
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Permutations and Combinations question

2025 · 24 Jan · Shift 1 · Q50

JEE MainMathematicsPermutations and CombinationsNumerical+4 / −1
The number of 3 -digit numbers, that are divisible by 2 and 3 , but not divisible by 4 and 9 , is ‾\underline{\hspace{2cm}}​.
Numerical answer
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Correct answer: 50

  1. We need the number of 3-digit numbers divisible by 222 and 333, but not divisible by 444 and 999.

  2. A number divisible by both 222 and 333 must be divisible by lcm⁡(2,3)=6.\operatorname{lcm}(2,3)=6.lcm(2,3)=6. So first count 3-digit multiples of 666.

  3. The 3-digit numbers run from 100100100 to 999999999.

The first 3-digit multiple of 666 is 102102102 and the last is 996996996. Thus the count is 996−1026+1=8946+1=149+1=150.\frac{996-102}{6}+1=\frac{894}{6}+1=149+1=150.6996−102​+1=6894​+1=149+1=150.

  1. Now exclude those divisible by 444 or 999. Since we are already among multiples of 666:
  • divisible by 666 and 444 means divisible by lcm⁡(6,4)=12\operatorname{lcm}(6,4)=12lcm(6,4)=12
  • divisible by 666 and 999 means divisible by lcm⁡(6,9)=18\operatorname{lcm}(6,9)=18lcm(6,9)=18

So from the 150 multiples of 666, remove 3-digit multiples of 121212 or 181818.

  1. Count 3-digit multiples of 121212: First is 108108108, last is 996996996. 996−10812+1=88812+1=74+1=75.\frac{996-108}{12}+1=\frac{888}{12}+1=74+1=75.12996−108​+1=12888​+1=74+1=75.

  2. Count 3-digit multiples of 181818: First is 108108108, last is 990990990. 990−10818+1=88218+1=49+1=50.\frac{990-108}{18}+1=\frac{882}{18}+1=49+1=50.18990−108​+1=18882​+1=49+1=50.

  3. Numbers counted in both are divisible by both 121212 and 181818, i.e. divisible by lcm⁡(12,18)=36.\operatorname{lcm}(12,18)=36.lcm(12,18)=36. Count 3-digit multiples of 363636: First is 108108108, last is 972972972. 972−10836+1=86436+1=24+1=25.\frac{972-108}{36}+1=\frac{864}{36}+1=24+1=25.36972−108​+1=36864​+1=24+1=25.

  4. By inclusion-exclusion, numbers divisible by 121212 or 181818 are 75+50−25=100.75+50-25=100.75+50−25=100.

  5. Therefore required count is 150−100=50.150-100=50.150−100=50.

  6. Equivalently, these are 3-digit numbers divisible by 666 but by neither 121212 nor 181818.

So the final integer answer is 50.\boxed{50}.50​.

  1. Comparison with stored answer:
  • Derived answer: 505050
  • Stored correct answer: 125125125

These do not match. The stored answer appears incorrect.

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