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Try this beautiful problem from Integer based on Prime number useful for ISI BStat Entrance.

## Prime number | ISI BStat Entrance | Problem no. 70

The number of integers $n>1$, such that n, n+2, n+4 are all prime numbers is ……

• Zero
• One
• Infinite
• More than one,but finite

### Key Concepts

Number theory

Algebra

Prime

But try the problem first…

Source

TOMATO, Problem 70

Challenges and Thrills in Pre College Mathematics

## Try with Hints

First hint

taking n=3, 5, 7, 11, 13, 17….prime numbers we will get

Case of n=3

n= 3

n+2=5

n+4=7

Case of n=5

then $n$=5

n+2=7

n+4=9 which is not prime….

Case of n=7,

then n=7

n+2=9 which is not prime …

n+4=11

Can you now finish the problem ……….

Second Hint

We observe that when n=3 then n,n+2,n+4 gives the prime numbers…..other cases all are not prime.Therefore any no can be expressed in anyone of the form 3k, 3k+1 and 3k+2.

can you finish the problem……..

Final Step

If n is divisible by 3 , we are done. If the remainder after the division by 3 is 1, the number n+2 is divisible by 3. If the remainder is 2, the number n+4 is divisible by 3

The three numbers must be primes! The only case n=3 and gives$(3,5,7)$