CA Foundation · Quantitative Aptitude · Sequence and Series
What is the arithmetic mean of the first 20 natural numbers?
The arithmetic mean of the first 20 natural numbers is 10.5. Their sum is 20×21/2 = 210, and dividing by 20 gives 10.5, which equals the average of the first and last terms, (1+20)/2.
- A10
- B10.5Correct
- C11
- D21
Explanation
Sum of first 20 natural numbers = 20×21/2 = 210. AM = 210/20 = 10.5. Check: AM of first and last term = (1+20)/2 = 10.5. The value 10 results from dividing by 21 wrongly or using n/2, and 21 is the sum of the first and last terms without halving.
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