The mean of first twenty positive even numbers is

The first twenty positive even numbers are 2, 4, 6, ..., 40. They form an arithmetic progression with first term 2, last term 40, and 20 terms. For an AP, the mean equals the average of the first and last terms, so the mean is (2 + 40) / 2 = 21. Option D is correct. You can check it by the sum: 2 times (1 + 2 + ... + 20) = 2 times 210 = 420, and 420 / 20 = 21.

• A (20): This is the count of terms, not the mean. It is also the 10th even number, not the 20th, so it is a common confusion.
• B (18) and C (17): These do not follow from the AP. Any correct calculation for the first twenty even numbers gives 21. A mean below 21 means a wrong last term or a miscount of terms.

Core concept: the mean of the first n even numbers is n + 1. This is a quick shortcut that reappears in data interpretation and quantitative aptitude questions.

Trap: aspirants often take the 20th even number as 20 instead of 40, and then get a mean around 10 to 11 or pick A because it matches the count.