Question

If the sum of first 20 positive even numbers is 420, what is the sum of first 20 positive odd numbers? Gmatclub

Answer 1

Answer: 400

Step-by-step explanation:

The first 20 odd and even numbers form an AP series

For first 20 odd numbers -

a = 1, d = 2, n = 20

Thus, aₙ = a + (n-1)d

             = 1 + (20-1)2

             = 1 + 38 = 39

Hence, the 20th positive odd number is 39.

Now to find the sum of n numbers -

Sₙ = ⁿ/2(a + aₙ)

⇒ S₂₀ = 20/2 (1 + 39)

          = 10 x 40

          = 400