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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