In how many ways 18 identical balls can be used in 15 different cricket matches?
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Each cricket match will require at least one ball, so we just to need to determine the number of ways of distributing the 3 "extra" (identical) balls amongst the 1 5 matches. With a k representing the number of extra balls required for match k , the equation we must solve is
k = 1 ∑ 1 5 a k = 3 for non-negative integers a k , 1 ≤ k ≤ 1 5 .
This is a stars and bars calculation, which according to Theorem two in the link has solution
( 3 3 + 1 5 − 1 ) = ( 3 1 7 ) = 6 8 0 .