What is the arithmetic mean of the first 20 odd natural numbers?
Answer: B
The first 20 odd natural numbers form an A.P.: 1, 3, 5, ..., 39. The sum of the first n odd numbers is \(n^2\). So the sum is \(20^2=400\). The arithmetic mean is Sum/Number of terms = \(400/20 = 20\). Alternatively, for an A.P., the mean is the average of the first and last term: \((1+39)/2 = 20\).