In a shooting competition, a man can score 0, 1, 2, 3, 4 or 5 points for each shot.
Find number of different ways in which he can score 30 points in exactly 7 shots.
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Can you explain your reasoning to arrive at the answer?
4 4 4 4 4 5 5 30 21
3 4 4 4 5 5 5 30 140
3 3 4 5 5 5 5 30 105
2 4 4 5 5 5 5 30 105
2 3 5 5 5 5 5 30 42
1 4 5 5 5 5 5 30 42
0 5 5 5 5 5 5 30 7
462
Possible arrangements of numbers are as follows: 1 ) 0 + 5 + 5 + 5 + 5 + 5 + 5 = 3 0 7 2 ) 4 + 4 + 4 + 4 + 4 + 5 + 5 = 3 0 ( 2 7 ) = 2 1 3 ) 3 + 3 + 4 + 5 + 5 + 5 + 5 = 3 0 ( 4 7 ) × 3 = 1 0 5 4 ) 2 + 4 + 4 + 5 + 5 + 5 + 5 = 3 0 ( 4 7 ) × 3 = 1 0 5 5 ) 3 + 4 + 4 + 4 + 5 + 5 + 5 = 3 0 ( 3 7 ) × 4 = 1 4 0 6 ) 2 + 3 + 5 + 5 + 5 + 5 + 5 = 3 0 ( 5 7 ) × 2 = 4 2 7 ) 1 + 4 + 5 + 5 + 5 + 5 + 5 = 3 0 ( 5 7 ) × 2 = 4 2
The result is: 7 + 2 1 + 1 0 5 + 1 0 5 + 1 4 0 + 4 2 + 4 2 = 4 6 2
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The number of ways he can score 30 points is the same as the number of ways he can be 5 points short of a perfect score.
The number of ways to distribute 5 points across 7 shots is ( 5 5 + 6 ) , using a "stars and bars" calculation. Since there are no restrictions on the arrangements (i.e. it is impossible for him to miss more than 5 points in a single shot, because he only missed 5 points total), the final answer is:
( 5 1 1 ) = 4 6 2