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

Revision as of 08:56, 18 June 2024

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 $\boxed{\textbf{(A)}\ 48}$ .

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 $n+8=40+8=\boxed{\mathrm{(A)}48}$

Solution 6

Let the number of games they won be $x$ and the number of games they lost be $y$ . We are told that $\frac{x}{x+y}=\frac{9}{20}$ , which can be manipulated to $20x=9x+9y$ which simplifies down to $11x=9y$ . Then, after district games, we are told $\frac{x+6}{x+y+8}=\frac12$ , which can be changed into $2x+12=x+y+8$ which simplifies down to $x+4=y$ . Then we can solve for $x$ using substitution: $11x=9x+36$ $2x=36$ $x=18$ Now that we know $x=18$ , we can figure out that $y=22$ . $18+22=40$ . Now we need to add on the district games: $40+8=\boxed{\textbf{(A)} 48 }$ .

Solution 7

Let $x$ be the total number of games before the district play. The Unicorns have $0.45x$ wins, therefore the rest $0.55x$ are losses. But after the district play, they won 6 and lost 2 more games. We can solve for x by forming the equation $0.45x+6=0.55x+2$ . Subtracting $2$ and $0.45x$ from both sides gives us $0.10x=4$ , and from here we multiply both sides by 10 to get $x=40$ . We are not finished yet as the problem is asking for the total games (which includes the games after the district play), so we add 8 to our value of x to get our answer which is $\boxed{\textbf{(A)} 48}$ .

- LearnForEver

Solution 8 (Answer Choices)

We can check each answer choice from left to right to see which one is correct. Suppose the Unicorns played $48$ games in total. Then, after district play, they would have won $24$ games. Now, consider the situation before district play. The Unicorns would have won $18$ games out of $40$ . Converting to a percentage, $18/40 = 45$ %. Thus, the answer is $\boxed{\textbf{(A)} 48}$ .

Note: If A didn't work, we would have similarly tested the other choices until we found one that did.

~cxsmi

Video Solution by OmegaLearn

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

~pi_is_3.14

@@ Line 9: / Line 9: @@
 <math>\frac{x}{y}=0.45</math> and <math>\frac{x+6}{y+8}=0.5.</math>
 We first multiply both sides of the first equation by <math>y</math> to get <math>x=0.45y.</math> Then, we multiply both sides of the second equation by <math>(y+8)</math> to get <math>x+6=0.5(y+8).</math> Applying the Distributive Property gives yields <math>x+6=0.5y+4.</math> Now we substitute <math>0.45y</math> for <math>x</math> to get <math>0.45y+6=0.5y+4.</math> Solving gives us <math>y=40.</math> Since the problem asks for the total number of games, we add on the last 8 games to get the solution <math>\boxed{\textbf{(A)}\ 48}</math>.
-==Solution 3==
-First we simplify <math>45</math>% to <math>\frac{9}{20}</math>. Ratio of won to total is <math>\frac{9}{20}</math>, but ratio of total number won to total number played is <math>\frac{9x}{20x}</math> for some <math>x</math>. After they won 6 more games and lost 2 more games the number of games they won is <math>9x+6</math>, and the total number of games is <math>20x+8</math>. Turning it into a fraction we get <math>\frac{9x+6}{20x+8}=\frac{1}{2}</math>, so solving for <math>x</math> we get <math>x=2.</math> Plugging in 2 for <math>x</math> we get <math>20(2)+8=\boxed{48}</math>.
 ==Solution 4==

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