Guys all of you know the formula n(n+1)/2 for sum of n natural numbers .

Here is some useful info to solve some typical prob quickly

The sum of 1st 10 natural numbers =55
sum of next 10 natural numbers(11-20)=155
sum of next i.e., (21-30)=255.................................it goes on 355, 455, 555, 655 etc

->sum of 1000 natural numbers=(100*101)/2=>5050 i.e.,(55+155+255+355+455+555+655+755+855+955)

ex:A boy starts adding consecutive natural numbers starting with 1, after some time he reaches a total of 1000, and realizes that he has made an error of double counting one number. find the double counted number.
ans:by using the above mentioned info
the sum of 1st 40 natural numbers=55+155+255+355=820
now you go on adding the remaining numbers till you reach near 1000
820+40+41+42+43+44=990
so the required ans is 10 (1000-990=10)
since double count will always have the effect of increasing the sum