Q. Find the missing number in the following number series. 3, 5, 8, 10, ?, 15, 18, 20
a) 13
b) 11
c) 14
d) 12
Correct Answer: a) 13
Question from UPPSC Prelims CSAT 2025
Explanation:
Observe the differences between consecutive terms: 5−3=2, 8−5=3, 10−8=2, then the pattern continues alternating +2, +3. So from 10, add 3 to get the missing term: 10+3=13.
Check forward: 13+2=15, 15+3=18, 18+2=20, which matches the given series. Hence, the missing number is 13.