CA Foundation · Quantitative Aptitude · Sequence and Series
The sum of the first n natural numbers is 210. Then the value of n is:
The value of n is 20. Setting n(n+1)/2 equal to 210 gives n(n+1) = 420, and the two consecutive integers whose product is 420 are 20 and 21, so n is 20.
- A21
- B20Correct
- C15
- D105
Explanation
n(n+1)/2 = 210 gives n(n+1) = 420 = 20×21, so n = 20. Option 21 is the larger factor, which is wrong because n is the smaller of the consecutive pair.
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