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Sets and Relations question

2022 · 29 Jul · Shift 1 · Q25
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  5. /2022 · 29 Jul · Shift 1 · Q25

Sets and Relations question

2022 · 29 Jul · Shift 1 · Q25

JEE MainMathematicsSets and RelationsMCQ+4 / −1
Let R be a relation from the set {1,2,3,…,60}\{1,2,3, \ldots, 60\}{1,2,3,…,60} to itself such that R={(a,b):b=pqR=\{(a, b): b=p qR={(a,b):b=pq, where p,q⩾3p, q \geqslant 3p,q⩾3 are prime numbers}. Then, the number of elements in R is :
  1. A
    600
  2. B
    660
  3. C
    540
  4. D
    720
View written solutionFree

Correct answer: B

  1. Understand the relation

We have a relation RRR from the set A={1,2,3,…,60}A=\{1,2,3,\ldots,60\}A={1,2,3,…,60} to itself, defined by R={(a,b):b=pq, where p,q≥3 are primes}.R=\{(a,b): b=pq,\text{ where } p,q\ge 3 \text{ are primes}\}.R={(a,b):b=pq, where p,q≥3 are primes}.

Here, aaa can be any element of AAA, while bbb must be an element of AAA that can be written as a product of two primes, each at least 333.

So the total number of ordered pairs in RRR is: ∣R∣=(number of choices for a)×(number of valid b).|R|=(\text{number of choices for }a)\times(\text{number of valid }b).∣R∣=(number of choices for a)×(number of valid b).

Since a∈Aa\in Aa∈A, there are clearly 606060 choices for aaa.

Thus we only need to count how many numbers b∈{1,2,…,60}b\in \{1,2,\dots,60\}b∈{1,2,…,60} can be written as b=pqb=pqb=pq with p,q≥3p,q\ge 3p,q≥3 prime.


  1. List primes at least 3 up to 60

The relevant primes are: 3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59.3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59.3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59.

We need products pq≤60pq\le 60pq≤60.


  1. Find all possible products pq≤60pq\le 60pq≤60

We allow p=qp=qp=q as well.

  • With p=3p=3p=3: \quad 3\cdot 5=15, \quad 3\cdot 7=21, \quad 3\cdot 11=33, \quad 3\cdot 13=39, \quad 3\cdot 17=51, \quad 3\cdot 19=57.$$ Next, $3\cdot 23=69>60$, so stop.
  • With p=5p=5p=5: \quad 5\cdot 7=35, \quad 5\cdot 11=55.$$ Next, $5\cdot 13=65>60$, so stop.
  • With p=7p=7p=7: 7⋅7=49.7\cdot 7=49.7⋅7=49. Next, 7⋅11=77>607\cdot 11=77>607⋅11=77>60, so stop.

For larger primes, even the square exceeds 606060, so no more values occur.

Hence the valid values of bbb are: {9,15,21,25,33,35,39,49,51,55,57}.\{9,15,21,25,33,35,39,49,51,55,57\}.{9,15,21,25,33,35,39,49,51,55,57}.

So the number of valid bbb is 11.11.11.


  1. Count ordered pairs

For each of the 111111 valid values of bbb, aaa can be any of the 606060 elements.

Therefore, ∣R∣=60×11=660.|R|=60\times 11=660.∣R∣=60×11=660.


  1. Check options

The correct option is 660\boxed{660}660​ which is Option B.


  1. Compare with stored answer

Stored correct answer: B

Our derived answer: B

So they agree.

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