Even divisibility!
How many positive even integers n are there such that 100<=n<=200 and n is neither divisible by seven nor by nine?
If you found them to be x ,then calculate:
(x+1)^2-(x-1)^2
The answer is 156.
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1 solution
Total no of even numbers between 100<=n<=200 is 51 and even neither divisible by 7 nor by 9 is 12(not divisible by 7 is 6 and not divisible by 9 is 6 and one is common i.e. divisible by 63). So x=39 and (x+1)^2-(x-1)^2=4x. Hence answer is 156