The missing number in the series 6, 11, 18, 27, 38, ---, 66 is:

The correct answer is C) 51. Take successive differences: 11-6=5, 18-11=7, 27-18=9, 38-27=11. The differences rise by 2 each time, so the next difference is 13 and the one after is 15. That gives 38+13=51, and 51+15=66, which matches the last term given.

Why the others fail:

• A) 41 gives a difference of only 3 after 38, which breaks the pattern of odd differences increasing by 2.
• B) 49 gives a difference of 11, repeating the earlier step instead of moving to 13. It also cannot reach 66 with the pattern that follows.
• D) 59 gives a jump of 21, which is far off the sequence, and 66 would then be only 7 more, breaking the rule.

The core concept is a second-order difference pattern: the first differences form an arithmetic progression. This kind of check is standard in the reasoning and numerical series section of the paper. It trains the habit of testing the difference series before guessing from the terms.

Trap: aspirants test only the first-order difference of the last two given terms and pick a number that looks close. Always write out every difference and confirm the pattern holds across the whole series before accepting an answer.