Question

what is the number of sequences of 1 and 2 such that they sum up to an non-negative integer n?"

Answer 1

Answer:

There are 9 × 10 4 = 90000 5-digit positive integers. Out of these 90000 positive integers, the sum of the digits of half of the numbers will add up to an odd number and the remaining half will add up to an even number. = 45000 5-digit positive integers whose sum add up to an odd number.

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