Question

What should come in place of the question mark (?) in the given series? 1, 2, 5, 12, ?, 58, 121

• A. 20
• B. 25
• C. 24
• D. 27

Answer 1

Answer:

D. 27

The formula for the $n$-th term is: $2^n - n$

• 1st term ($n=1$): $2^1 - 1 = 2 - 1 = 1$

• 2nd term ($n=2$): $2^2 - 2 = 4 - 2 = 2$

• 3rd term ($n=3$): $2^3 - 3 = 8 - 3 = 5$

• 4th term ($n=4$): $2^4 - 4 = 16 - 4 = 12$

• 5th term ($n=5$, Missing Number): $2^5 - 5 = 32 - 5 =$ 27

• 6th term ($n=6$): $2^6 - 6 = 64 - 6 = 58$

• 7th term ($n=7$): $2^7 - 7 = 128 - 7 = 121$