Question
Find the sum of all the numbers between 200 and 400 which are divisible by 7.
Solution
All the numbers between 200 and 400, which are divisible by 7 are:
203, 210, 217 ......, 399.
Here, a = 203, d = 210 - 203 = 7, 
or a + (n - 1)d = 399
203 + (n - 1) 7 = 399
(n - 1) 7 = 399 - 203 = 196 
∴ 
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