Shooting Mania

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.

Image Credit: Wikimedia IIVQ .


The answer is 462.

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4 solutions

Kerry Soderdahl
Jul 7, 2015

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 + 6 5 ) {{5+6}\choose{5}} , 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:

( 11 5 ) = 462 {11\choose{5}}=\boxed{462}

Murat Yoğurtçu
Jul 2, 2015

(11,6)=462

Moderator note:

Can you explain your reasoning to arrive at the answer?

Lu Chee Ket
Oct 5, 2015

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
Maria Kozlowska
Jul 5, 2015

Possible arrangements of numbers are as follows: 1 ) 0 + 5 + 5 + 5 + 5 + 5 + 5 = 30 7 1) \quad 0 + 5 + 5 + 5 + 5 + 5 + 5 = 30 \qquad 7 \qquad \qquad \qquad 2 ) 4 + 4 + 4 + 4 + 4 + 5 + 5 = 30 ( 7 2 ) = 21 2) \quad 4 + 4 + 4 + 4 + 4 + 5 + 5 = 30 \qquad {7 \choose 2} = 21 \qquad 3 ) 3 + 3 + 4 + 5 + 5 + 5 + 5 = 30 ( 7 4 ) × 3 = 105 3) \quad 3 + 3 + 4 + 5 + 5 + 5 + 5 = 30 \qquad {7 \choose 4} \times 3 = 105 4 ) 2 + 4 + 4 + 5 + 5 + 5 + 5 = 30 ( 7 4 ) × 3 = 105 4) \quad 2 + 4 + 4 + 5 + 5 + 5 + 5 = 30 \qquad {7 \choose 4} \times 3 = 105 5 ) 3 + 4 + 4 + 4 + 5 + 5 + 5 = 30 ( 7 3 ) × 4 = 140 5) \quad 3 + 4 + 4 + 4 + 5 + 5 + 5 = 30 \qquad {7 \choose 3} \times 4 = 140 6 ) 2 + 3 + 5 + 5 + 5 + 5 + 5 = 30 ( 7 5 ) × 2 = 42 6) \quad 2 + 3 + 5 + 5 + 5 + 5 + 5 = 30 \qquad {7 \choose 5} \times 2 = 42 7 ) 1 + 4 + 5 + 5 + 5 + 5 + 5 = 30 ( 7 5 ) × 2 = 42 7) \quad 1 + 4 + 5 + 5 + 5 + 5 + 5 = 30 \qquad {7 \choose 5} \times 2 = 42

The result is: 7 + 21 + 105 + 105 + 140 + 42 + 42 = 462 7 + 21 + 105 + 105 + 140 + 42 + 42 = \boxed{462}

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