Question

The sum of all odd numbers of two digits is

• A. 2475
• B. 2530
• C. 4905
• D. 5049

Answer 1

Answer: (A)

2475

Answer 2

Required sum, $S = 11 + 13 + 15 + ....... + 99$

Let the number of odd terms be n, then $99 = 11 + (n - 1)2$

⇒ $n = 45$

∴ $S = \frac { 45 } { 2 } ( 11 + 99 ) = 45 \times 55 = 2475$ $\left\lbrack \because S = \frac{n}{2}(a + l) \right\rbrack$