We have a deck of 90 cards that are numbered from 10 to 99 (all two-digit numbers). How many sets of three or more different cards in this deck are there such that the number on one of them is the sum of the other numbers, and those other numbers are consecutive?
Solution
We are looking at sums of the form
where ( represents the starting number of the consecutive numbers in a set) and represents the number of consecutive numbers in a set. Each of these sums (the -th number in the set) must be less than, or equal to, . By taking , we see that the maximum possible value for is .
For the case (i.e., where three cards are used), we must have , so that . The possibilities of three cards are: .
For the case (i.e., where four cards are used), we get , so that , representing the groups of four cards: .
Continuing in this way, we find that when , when , when , and when . All in all, there are sets of cards satisfying the conditions.
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