Question 10
The sum of all the natural numbers between 301 and 501 (including 301 and 501) which are divisible by 7 is
Solution
301 is divisible by 7 (43 times)
497 is also divisible by 7 (71 times)
So, the required sum is
7 ( 43 + 44 + .... + 71)
= 7 x (sum of numbers upto 71 - sum of numbers upto 42) = 7 * [(71x 72/2)-(42 x 43/2)]
= 11571
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