The value of 1+2+3+...+30+31+30+29+...+3+2+1 =?

Correct option is A

Given:
$$​1+2+3+...+30+31+30+29+...+3+2+1 =$$​​
Formula Used:
Sum of first n natural numbers = n × (n + 1)/2

Solution:
(1 + 2 + 3 + ........... + 29 + 30+ 31) + (30 + 29 + 28 + .......... + 3 + 2 + 1)
=> $$\left[\frac{31 \times (31 + 1)}{2}\right] + \left[\frac{30 \times (30 + 1)}{2}\right]$$​​

=> (16 × 31) + (31 × 15)
=> 31 (16 + 15)

=> 961