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Consider all natural numbers from 1 to 101, inclusive.
Their amount is 101.
From this amount, exclude all even natural numbers from 1 to 100 --- their amount is = 50.
The difference 101 - 50 = 51 is your answer.
Solved.
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This problem is accessible for the 3-rd - 4-th young students,
who do not know yet neither arithmetic sequences nor equations.
Nevertheless, this problem is a good exercise for the young minds.
So, the solution to the problem must be accessible for them, too.
In other words, the solution should be adequate to the problem.
It is why I came with my post.