A positive integer is picked randomly from 61 to 70, inclusive.

What is the probability that it is prime?

Question

A positive integer is picked randomly from 61 to 70, inclusive.

What is the probability that it is prime?

anonymous · Answer

pls explain the concept of prime no

anonymous · Answer

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

anonymous · Answer

do you know what "prime number " means?

anonymous · Answer

i m confused

hoblos · Answer

the prime numbers are 61 & 67 only

anonymous · Answer

lol prime is not odd!

anonymous · Answer

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

hoblos · Answer

so the probability is 2/10 = 1/5

anonymous · Answer

we can try the numbers one by one and check

anonymous · Answer

okay,pratu u were mistaken

anonymous · Answer

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

anonymous · Answer

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

anonymous · Answer

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

anonymous · Answer

why did u take such a big no?

anonymous · Answer

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

anonymous · Answer

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 ?

anonymous · Answer

true

anonymous · Answer

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.

anonymous · Answer

well sorry i m a lost in maths

anonymous · Answer

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

hoblos · Answer

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