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Algebra Difficulty 6.9 National Olympiad Prove it Bulgaria

Problem:

Four players A1A_{1}, A2A_{2}, A3A_{3} and A4A_{4} have the same amounts of money and play the following game with seven dices: A1A_{1} throws the seven dices and then pays to each of the other three players 1k\frac{1}{k} of the money that the corresponding player has at the moment, where kk is the sum of the points on the seven dices. Then the same action is performed consecutively by A2A_{2}, A3A_{3} and A4A_{4} and the game is over. Find the sums of the points on the dices thrown by each player if after the game their money are in ratio 3:3:2:23: 3: 2: 2 (the money of A1A_{1} to the money of A2A_{2} to the money of A3A_{3} to the money of A4A_{4} ).

Solution

Solution:

Denote by Sk(m)S_{k}^{(m)} the money of the kk-th player, k=1,2,3,4k=1,2,3,4, after the move and payment of the mm-th, m=1,2,3,4m=1,2,3,4, and Sk(0)=SS_{k}^{(0)}=S in the beginning, k=1,2,3,4k=1,2,3,4. Denote by aia_{i} the sum of points on the dices thrown by AiA_{i}. It follows from the game rules that Sk(m)=Sk(m1)+1amSk(m1)=Sk(m1)1+amamS_{k}^{(m)}=S_{k}^{(m-1)}+\frac{1}{a_{m}} S_{k}^{(m-1)}=S_{k}^{(m-1)} \frac{1+a_{m}}{a_{m}} for kmk \neq m (i.e. when AkA_{k} gets money) and
Sk(k)=Sk(k1)1akikSi(k1)=Sk(k1)1ak(4SSk(k1))=Sk(k1)1+akak4Sak \begin{aligned} S_{k}^{(k)} & =S_{k}^{(k-1)}-\frac{1}{a_{k}} \sum_{i \neq k} S_{i}^{(k-1)}=S_{k}^{(k-1)}-\frac{1}{a_{k}}\left(4 S-S_{k}^{(k-1)}\right) \\ & =S_{k}^{(k-1)} \frac{1+a_{k}}{a_{k}}-\frac{4 S}{a_{k}} \end{aligned}
when AkA_{k} pays.

Using these formulas we find the money of the four players in the end of the game. We obtain that
6S5=S1(4)=PS4S(1+a2)(1+a3)(1+a4)a1a2a3a46S5=S2(4)=PS4S(1+a3)(1+a4)a2a3a44S5=S3(4)=PS4S(1+a4)a3a44S5=S4(4)=PS4Sa4 \begin{aligned} \frac{6 S}{5} & =S_{1}^{(4)}=P S-\frac{4 S\left(1+a_{2}\right)\left(1+a_{3}\right)\left(1+a_{4}\right)}{a_{1} a_{2} a_{3} a_{4}} \\ \frac{6 S}{5} & =S_{2}^{(4)}=P S-\frac{4 S\left(1+a_{3}\right)\left(1+a_{4}\right)}{a_{2} a_{3} a_{4}} \\ \frac{4 S}{5} & =S_{3}^{(4)}=P S-\frac{4 S\left(1+a_{4}\right)}{a_{3} a_{4}} \\ \frac{4 S}{5} & =S_{4}^{(4)}=P S-\frac{4 S}{a_{4}} \end{aligned}
where
P=(1+a1)(1+a2)(1+a3)(1+a4)a1a2a3a4 P=\frac{\left(1+a_{1}\right)\left(1+a_{2}\right)\left(1+a_{3}\right)\left(1+a_{4}\right)}{a_{1} a_{2} a_{3} a_{4}}
By the first two equations we get a2=a11a_{2}=a_{1}-1, and by the last two we have a4=a31a_{4}=a_{3}-1. Now by the second and third equations we obtain
25=4a44(1+a3)a2a4(a2+10)(10a4)=120 \frac{2}{5}=\frac{4}{a_{4}}-\frac{4\left(1+a_{3}\right)}{a_{2} a_{4}} \Longleftrightarrow\left(a_{2}+10\right)\left(10-a_{4}\right)=120
It follows from the later equation that a4<10a_{4}<10. We also have a47a_{4} \geq 7 since the dices are 7 and therefore the minimum sum is 7. It remains to check the possibilities a4=7,8a_{4}=7,8 and 99. The only solution appears for a4=7a_{4}=7 (the maximum sum is 42) which gives a3=8a_{3}=8, a2=30a_{2}=30 and a1=31a_{1}=31.

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