Which number will come at the place of the question mark (?) in the given series?

Look at the pattern:

8 → 18 → 38 → 78 → 158

Check the rule:
$8 \times 2 + 2 = 18$
$18 \times 2 + 2 = 38$
$38 \times 2 + 2 = 78$
$78 \times 2 + 2 = 158$

So the pattern is: multiply by 2 and add 2

Next term:
$158 \times 2 + 2 = 316 + 2 = 318$

Correct answer: 318