Find the sum of all natural number from 1 to 1000 which are not divisible by 5
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sum of first 'n' natural numbers =n(n+1)/2
sum of any 'n' numbers =n/2[1st term + last term]
sum of 1st 1000 natural numbers =1000×(1001)/2
=500500-----------------(1)
total number of 5 divisibles in 1-1000 is
1000/5=200 numbers
[5,10,15,......1000] is total 200 numbers
sum of all 5divisible is
[5+10+15+......+1000] =(200)/2×[5+1000]=100500----------(2)
{1st number is 5,last number is 1000,total numbers n=200}
(1)-(2)
we get ans = 400000
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