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## anonymous 4 years ago A positive integer is picked randomly from 61 to 70, inclusive. What is the probability that it is prime?

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1. anonymous

pls explain the concept of prime no

2. anonymous

The numbers from 61 to 70 are: 61, 62, 63, 64, 65, 66, 67, 68, 69, 70 Prime numbers: 61, 63, 67, 69

3. anonymous

do you know what "prime number " means?

4. anonymous

i m confused

5. hoblos

the prime numbers are 61 & 67 only

6. anonymous

lol prime is not odd!

7. anonymous

prime means only divisors are 1 and itself, a whole number that you cannot factor

8. hoblos

so the probability is 2/10 = 1/5

9. anonymous

we can try the numbers one by one and check

10. anonymous

okay,pratu u were mistaken

11. anonymous

61, 62, 63, 64, 65, 66, 67, 68, 69, 70 61 is prime because it is not the product of any two numbers. think about the multiplication table. 61 does not appear as an entry there

12. anonymous

62 is even, so it is divisible by 2, not prime. and neither are 64,66,68,70 for the same reason

13. anonymous

63 is divisible by 3 because $63=3\times 21$

14. anonymous

why did u take such a big no?

15. anonymous

65 is obviously divisible by 5 67 is prime and finally 69 is divisible by 3

16. anonymous

I think a better question is how do you know if a number is a prime. We know that all even numbers are not prime. But which of the odd ones are prime. I guess the question is how would you calculate if a number is or is not prime? For example, 3335679 ?

17. anonymous

true

18. anonymous

Can this only be done by brute force. i.e. take each integer and start dividing by the smaller integers and then keep going until you reach an answer without a remainder? We need a mathematician.

19. anonymous

well sorry i m a lost in maths

20. anonymous

Yeah, I made a mistake and had some internet problem in the middle so couldn't reply. The others are right though.

21. hoblos

some notes that may help: if the number is even then it is divisible by 2 if the sum of digits of the number is divisible by 3 then the whole number is divisible by 3 if the number ends with 0 or 5 then it is divisible by 5 out of these cases you have to try dividing by PRIME numbers until you reach an answer without a remainder

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