Find the sum of all the natural numbers from 21 to 50.

The correct answer is Option (1) → 1065

The numbers 21, 22, ..., 50 go up by 1 each time, so they form an arithmetic progression. The sum of such a progression is the number of terms times the average of the first and the last term, because the terms pair off from the two ends (21 + 50, 22 + 49, ...) with the same total.

• Number of terms from 21 to 50: \(50-21+1=30\)

• Sum \(=\displaystyle\frac{30}{2}\times(21+50)=15\times71=1065\)