Find the missing number in the series: 3 5 11 21 43 __

Key idea: double each term and alternately subtract 1 then add 1.

• 3 × 2 − 1 = 5

• 5 × 2 + 1 = 11

• 11 × 2 − 1 = 21

• 21 × 2 + 1 = 43

• 43 × 2 − 1 = 85

Therefore the missing number is 85.