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Combinatorics Difficulty 6.0 National olympiad Find the answer China

Before The World Cup tournament, the football coach of FF country will let seven players, A1,A2,,A7A_1, A_2, \dots, A_7, join three training matches (90 minutes each) in order to assess them. Suppose, at any moment during a match, one and only one of them enters the field, and the total time (which is measured in minutes) on the field for each one of A1,A2,A3A_1, A_2, A_3 and A4A_4 is divisible by 77 and the total time for each of A5,A6A_5, A_6 and A7A_7 is divisible by 1313. If there is no restriction about the number of times of substitution of players during each match, then how many possible cases are there within the total time for every player on the field?

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Suppose that xix_i (i=1,2,,7i = 1, 2, \dots, 7) minutes is the time for ii-th player on the field. Now, the problem is to find the number of solution groups of positive integers for the following equation:
x1+x2++x7=270 x_1 + x_2 + \cdots + x_7 = 270
when the conditions 7xi7 \mid x_i (i=1,2,3,4i = 1, 2, 3, 4) and 13xj13 \mid x_j (j=5,6,7j = 5, 6, 7) are satisfied.

Suppose x1+x2+x3+x4=7mx_1 + x_2 + x_3 + x_4 = 7m and x5+x6+x7=13nx_5 + x_6 + x_7 = 13n. Then
7m+13n=270, 7m + 13n = 270,
and m,nN+,m4m, n \in \mathbb{N}_+, m \ge 4 and n3n \ge 3.

When (m,n)=(33,3)(m, n) = (33, 3), x5=x6=x7=13x_5 = x_6 = x_7 = 13. Let xi=7yix_i = 7y_i (i=1,2,3,4i = 1, 2, 3, 4), then
y1+y2+y3+y4=33. y_1 + y_2 + y_3 + y_4 = 33.
We get C3314=C323=4960C_{33-1}^4 = C_{32}^3 = 4960 solution groups of positive integers (y1,y2,y3,y4)(y_1, y_2, y_3, y_4) and in this case, we have 49604960 solution groups of positive integers satisfying the conditions.

When (m,n)=(20,10)(m, n) = (20, 10), let xi=7yix_i = 7y_i (i=1,2,3,4i = 1, 2, 3, 4) and xj=13yjx_j = 13y_j (j=5,6,7j = 5, 6, 7). Hence
y1+y2+y3+y4=20 and y5+y6+y7=10. y_1 + y_2 + y_3 + y_4 = 20 \text{ and } y_5 + y_6 + y_7 = 10.
In this case, we have C193×C93=34884C_{19}^3 \times C_9^3 = 34884 solution groups of positive integers satisfying the conditions.

When (m,n)=(7,17)(m, n) = (7, 17), set xi=7yix_i = 7y_i (i=1,2,3,4i = 1, 2, 3, 4) and xj=13yjx_j = 13y_j (j=5,6,7j = 5, 6, 7). Hence
y1+y2+y3+y4=7 and y5+y6+y7=17. y_1 + y_2 + y_3 + y_4 = 7 \text{ and } y_5 + y_6 + y_7 = 17.
In this case, we have C63×C163=2400C_6^3 \times C_{16}^3 = 2400 solution groups of positive integers satisfying the conditions.

Consequently, for (1)(1), there are
4960+34884+2400=42244 4960 + 34884 + 2400 = 42244
solution groups of positive integers satisfying the conditions.

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