Aptitude - Odd Man Out and Series - Discussion

81 is divisible by 9, so it isn't a prime number. 9x9=81. the rest of the numbers can not be divided into evenly without a remainder. For example 5 is a prime because if you divide it by 2 you will have a remainder. 5 is only divisible by 5 or by 1.

also stated above was the formula: tn= t(n-1)+2n
where n= 1,2,3,4,...etc.
and t= 41,43,47,53,...etc.

If you plug in the numbers it doesn't equal the 2nd term which is 43.
let t= 1st term 41
n= 1

Example with above formula:tn= 41(1-1)+2(1)
41(0)+2 = 2?
I do understand t+2n
41+2(1)= 43
43+2(2)= 47
47+2(3)= 53

I am not trying to be picky, I was really trying to understand.
thanks for the above solutions!

Thank you ankita.

Hey people it should be 73 also..... because in each case 2 is getting added and in that case 73 doesn't fit!!

What is prime no?

4*10+1, 7*6+1, 6*8-1, 6*9-1, 6*10+1, 7*10+1, 8*9+1,
but there are 9*9= 81

I am not agree with Vikash and ankita as according to them 73 and 81 both will be the answer.But we have to choose one answer.So correct answer is 81 which is not a prime number

I agree with praveen.

Hey how you express above formula in to the sum. tn=t (n-1) +2n.
For example =41 (1-1) +2*1=2.

41+2=43.
43+4=47.
47+6=53.
53+8=61.
61+10=71.
71+2=73.
73+4=77.
But there is 83, which was not in same sequence. There must be 77.

A number which is divisible by one (i.e 1) and itself (the same number that is given) is called a prime number.