Goalkeeper!

Algebra Level 2

During a football/soccer match, a goalkeepers save rate was 33 1 3 % . 33\frac{1}{3}\%.

After saving the next shot, it rose to 40 % . 40\%. How many more consecutive shots on target must be saved to raise the save rate to 50 % ? 50\%?

4 3 1 2

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2 solutions

Eli Ross Staff
Oct 29, 2015

After n n shots, the goalkeeper had saved n 3 \frac{n}{3} of them, so after saving the next shot, he has saved n 3 + 1 = 40 % × ( n + 1 ) \frac{n}{3}+1 = 40\% \times (n+1) of them. Solving the equation n 3 + 1 = 2 n 5 + 2 5 \frac{n}{3}+1 = \frac{2n}{5} + \frac{2}{5} gives n = 9. n=9.

In other words, he originally saved 3 of 9 shots. When he saved the next shot, he had saved 4 of 10. Now, if he saves s s consecutive shots, his save rate is 4 + s 10 + s . \frac{4+s}{10+s}. Setting this equal to 50% gives 2 ( 4 + s ) = 10 + s , 2(4+s) = 10+s, so s = 2. s=2.

In other words, if he saves the next 2 shots, he will have saved 6 of 12!

Prasit Sarapee
Nov 3, 2015

33.333% = 1/3 = 1 save in 3 shots.

So 33.333%=(1/3) ==> (1+1)/(3+2) = 2/5 = 40%.

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