Hard
Find the missing term in the series: 3, 8, 18, 38, 78, ?
Explanation
Explanation
Step 1: The terms roughly double each time, so test a rule of the form "× 2 + k".
Step 2: 3 × 2 + 2 = 8 ✓, 8 × 2 + 2 = 18 ✓, 18 × 2 + 2 = 38 ✓, 38 × 2 + 2 = 78 ✓.
Step 3: The rule is "multiply by 2 and add 2".
Step 4: Next term = 78 × 2 + 2 = 156 + 2 = 158.
Answer: 158.
PRO TIP
Re-check your working by substituting the answer back into the original question.