How do you calculate 1+2+3+4+5+6+7+8+9+10 quickly?

Solution:
The generalized formula to find the sum of first N natural numbers is
(N * (N + 1) ) /2
For the series given, the value of N is 10
So substituting value of N in the generalized formula
( 10 * (10 + 1) ) / 2
=> (10 * 11 ) / 2
=> 110/2
=> 55
Hence the answer is 55