Difference between revisions of "2007 AMC 8 Problems/Problem 20"

Revision as of 17:14, 5 January 2023

Problem

Before the district play, the Unicorns had won $45$ % of their basketball games. During district play, they won six more games and lost two, to finish the season having won half their games. How many games did the Unicorns play in all?

$\textbf{(A)}\ 48\qquad\textbf{(B)}\ 50\qquad\textbf{(C)}\ 52\qquad\textbf{(D)}\ 54\qquad\textbf{(E)}\ 60$

Solution 1

At the beginning of the problem, the Unicorns had played $y$ games and they had won $x$ of these games. From the information given in the problem, we can say that $\frac{x}{y}=0.45.$ Next, the Unicorns win 6 more games and lose 2 more, for a total of $6+2=8$ games played during district play. We are told that they end the season having won half of their games, or $0.5$ of their games. We can write another equation: $\frac{x+6}{y+8}=0.5.$ This gives us a system of equations: $\frac{x}{y}=0.45$ and $\frac{x+6}{y+8}=0.5.$ We first multiply both sides of the first equation by $y$ to get $x=0.45y.$ Then, we multiply both sides of the second equation by $(y+8)$ to get $x+6=0.5(y+8).$ Applying the Distributive Property gives yields $x+6=0.5y+4.$ Now we substitute $0.45y$ for $x$ to get $0.45y+6=0.5y+4.$ Solving gives us $y=40.$ Since the problem asks for the total number of games, we add on the last 8 games to get the solution $\boxed{\textbf{(A)}\ 48}$ .

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Solution 2

Simplifying 45% to $\frac{9}{20}$ , we see that the numbers of games before district play are a multiple of 20. After that the Aces played 8 more games to the total number of games is in the form of 20x+8 where x is any positive integer. The only answer choice is $\boxed{48}$ , which is 20(2)+8.

Solution 3

First we simplify $45$ % to $\frac{9}{20}$ . Ratio of won to total is $\frac{9}{20}$ , but ratio of total number won to total number played is $\frac{9x}{20x}$ for some $x$ . After they won 6 more games and lost 2 more games the number of games they won is $9x+6$ , and the total number of games is $20x+8$ . Turning it into a fraction we get $\frac{9x+6}{20x+8}=\frac{1}{2}$ , so solving for $x$ we get $x=2.$ Plugging in 2 for $x$ we get $20(2)+8=\boxed{48}$ .

Solution 4

Because 45% can be simplified to $9/20$ , and we know that we cannot play a fraction amount of games, we know that the amount of games before district play is divisible by 20. After district play, there was $8$ games, so in total there must be $20x+8$ . The only answer in this format is $\boxed{\mathrm{(A)}48}$ .

Solution 5

Let $n$ be the number of pre-district games. Therefore, we can write the percentage of total games won as a weighted average, namely $.45(n)+.75(8)=(n+8)(.5)$ . Solving this equation for $n$ gives $40$ , but since the problem asked for all games, the answer is $n+8=40+8=\boxed{\mathrm{(A)}48}$

Video Solution by OmegaLearn

https://youtu.be/rQUwNC0gqdg?t=1993

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 ==Solution 2==

2007 AMC 8 (Problems • Answer Key • Resources)
Preceded by Problem 19	Followed by Problem 21
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