Aptitude - Odd Man Out and Series - Discussion

All the numbers are prime numbers except 81.

Prime number series is 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251, 257, 263, 269, 271, 277, 281, 283, 293, 307, 311, 313, 317, 331, 337, 347, 349, 353, 359, 367, 373, 379, 383, 389, 397, 401, 409, 419, 421, 431, 433, 439, 443, 449, 457, 461, 463, 467, 479, 487, 491, 499, 503, 509, 521, 523, 541, 547, 557, 563, 569, 571, 577, 587, 593, 599, 601, 607, 613, 617, 619, 631, 641, 643, 647, 653, 659, 661, 673, 677, 683, 691, 701, 709, 719, 727, 733, 739, 743, 751, 757, 761, 769, 773, 787, 797, 809, 811, 821, 823, 827, 829, 839, 853, 857, 859, 863, 877, 881, 883, 887, 907, 911, 919, 929, 937, 941, 947, 953, 967, 971, 977, 983, 991, 997 etc.

Except 81 is multiplied wit 9 & other numbers can be multiplied by itself.

81 is not a prime number.

A prime number is which is divisible by 2 or itself & in this case 81 (9x9 = 81) and the answer is 73 okay.

Answer is 73 not 81, But 81 should be 83.

73 is wrong in the series according to me. 2 is getting added.

41, 43, 47, 53, 61, 71, 73, 81.
The difference b/w two numbers is even series. i.e, 2,4,6,8,10.

But 73-71 = 2.
So we can get 73 as wrong number of the series.

@Soourabh.

73-71=12?
Can you explain it?

41, 43, 47, 53, 61, 71, 73, 81.

Difference b/w to number is even series..

43-41 = 2.
47-43 = 4.
53-47 = 6.
61-53 = 8.
71-61 = 10.
73-71 = 12.
81-73 = 9 its wrong so answer is (D).

The prediction is right since,

I too tried by the logic such as:

41+2 = 43.
43+4 = 47.
47+6 = 53.
53+8 = 61.
61+10 = 71........71+2 = 73.

In this case when multiplier become 12 then the logic is violated so if we follow this prediction the answer should be 73, so I think this not proper prediction, and the prediction that mentioned in the key even proper than this.
So that is the pragmatic logic.

81 is not a prime number but it is a perfect square.