Maths Olympiad Prep

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Algebra Difficulty 6.4 National Olympiad Prove it South Africa

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
s(10+,k)=i=0k1(10++i)=k(10++12(k1)), s(10 + \ell, k) = \sum_{i=0}^{k-1} (10 + \ell + i) = k\left(10 + \ell + \frac{1}{2}(k-1)\right),
where 0\ell \ge 0 (10+10 + \ell represents the starting number of the consecutive numbers in a set) and k2k \ge 2 represents the number of consecutive numbers in a set. Each of these sums (the (k+1)(k+1)-th number in the set) must be less than, or equal to, 9999. By taking =0\ell = 0, we see that the maximum possible value for kk is 77.

For the case k=2k = 2 (i.e., where three cards are used), we must have 2(10++12(21))992 \cdot (10+\ell+\frac{1}{2}(2-1)) \le 99, so that 39\ell \le 39. The 39+1=4039+1=40 possibilities of three cards are: 10+11=21,11+12=23,,49+50=9910+11=21, 11+12=23, \dots, 49+50=99.

For the case k=3k = 3 (i.e., where four cards are used), we get 3(10++12(31))993 \cdot (10+\ell+\frac{1}{2}(3-1)) \le 99, so that 22\ell \le 22, representing the 22+1=2322+1=23 groups of four cards: 10+11+12=33,,32+33+34=9910+11+12=33, \dots, 32+33+34=99.

Continuing in this way, we find that 13\ell \le 13 when k=4k = 4, 7\ell \le 7 when k=5k = 5, 4\ell \le 4 when k=6k = 6, and 1\ell \le 1 when k=7k = 7. All in all, there are 40+23+14+8+5+2=9240+23+14+8+5+2=92 sets of cards satisfying the conditions.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.