Question

What should come in place of ‘?’ in the given series? 105, 93, 78, 111, 91, 81, 117, 89, 84, ?

• A. 119
• B. 115
• C. 123
• D. 87

Answer 1

Answer:

C. 123

The number that should come in place of ‘?’ is 123.

The given sequence is a combination of three alternating series interleaved together:

• Series 1 (1st, 4th, 7th, and 10th terms): Adds 6 each time.

  • $105 \xrightarrow{+6} 111 \xrightarrow{+6} 117 \xrightarrow{+6} \mathbf{123}$

• Series 2 (2nd, 5th, and 8th terms): Subtracts 2 each time.

  • $93 \xrightarrow{-2} 91 \xrightarrow{-2} 89$

• Series 3 (3rd, 6th, and 9th terms): Adds 3 each time.

  • $78 \xrightarrow{+3} 81 \xrightarrow{+3} 84$

Following the pattern of Series 1, the next number is $117 + 6 = 123$.