Question

5, 11, 21, 43, 85, ?

• A. 185
• B. 170
• C. 171
• D. 181

Hint:

Look for alternating \(\pm 1\) after a common multiplier.

Answer 1

The Correct Option is C

The pattern doubles and alternately adds/subtracts 1: \[ 5\!\times\!2{+}1=11,\quad 11\!\times\!2{-}1=21,\quad 21\!\times\!2{+}1=43,\quad 43\!\times\!2{-}1=85. \] Next: \[ 85\!\times\!2{+}1=171. \] \[ \boxed{171} \]