Question 8

What is the sum of all natural numbers between 100 and 400 which are divisible by 13?

Solution

13*8=104
13*30=390
So sum =104+117+.....390
a=104
l=390
number of terms=((390-104)/13)+1
=22+1
=23
Sum=n(a+l)/2
=23(390+104)/2
=247*23
=5681

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