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# Tutor profile: Chris B.

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Chris B.
Undergraduate Student at Johns Hopkins University. Guitar Player for 9 Years
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## Questions

### Subject:Pre-Algebra

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Question:

You're making cookies with your grandmother for your whole family as a dessert after dinner. There are going to be 12 people at dinner, and you have 5 cookies in the oven baking currently. How many more cookies do you need to make so that everyone at dinner can have a cookie for dessert?

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Chris B.

### Subject:Music

TutorMe
Question:

How do you play a C chord on the guitar?

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Chris B.

### Subject:Algebra

TutorMe
Question:

Say you have \$200 to spend. You go to the electronics store and see that video games cost \$20 and movies cost \$15. You leave the store with 11 items combined and \$5 left over. How many of each item did you buy?

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Chris B.

In order to solve this problem, we must use a system of equations. Let's start by setting the number of video games we bought equal to the variable x, and the number of movies we bought equal to the variable y. Since we know that we have 11 total items, we can write the following equation: x + y = 11. This is a good start, but we still have two unknowns! We need another equation. Let's start this one by figuring out how much money we spent in total. Subtracting the amount of money we started with from the amount of money we walk out of the store with, we see that \$200 - \$5 = \$195 So we spent \$195 at the store. Additionally, we know from the information in the problem that video games cost \$20 and movies cost \$15. Remembering that the variable x = the number of video games we bought and y = the number of movies we bought, we can write down a very similar equation to our first equation and say that 20x + 15y = 195 We can make this statement because multiplying x, the number of videogames we bought, by 20, the cost of one video game, will give us the total amount that we spent on all of the video games, and multiplying y, the number of movies we bought, by 15, the cost of the movie, will give us the total amount we spent on all of the movies. Adding these two numbers will give us 195, the total amount we spent at the store. Now it's time to solve our system of equations. I find it easiest to start with the first equation, x + y = 11. By subtracting y from both sides, we can isolate the variable x and obtain the following equation: x = 11 - y Now that we have isolated the variable x, we can plug in this value of x to our second equation, 20x + 15y = 195. Doing so gives us the following equation: 20(11-y) + 15 y = 195. Now we have an equation with only 1 unknown! This means we can solve for y. The first step is to distribute the 20 into each number in the parentheses, which means we must multiply. This gives us the equation 220 - 20y + 15 y = 195. Now we want to isolate y on one side of the equation, so we must combine the two y terms and subtract the 220 over to the other side. -20y + 15y = -5y, and 195 - 220 = 25. So, we can rewrite our equation as -5y = -25 Now, all we have to do is divide the -5 away from the y, and we have our answer! So, y = -25/-5 a negative number divided by a negative number is a positive number, so y = 25/5 25/5 = 5, so we can say that y = 5! We're halfway there! Now that we have our value of y, we can obtain our value of x by plugging in our value of y into one of our first two equations. Let's pick the first one, as it is a simpler equation overall. Again, that first equation we wrote down is x + y = 11 So, because we know that y = 5, we plug 5 into that equation wherever we see y. This gives us the equation x + 5 = 11. Now all we have to do to find our value for x is subtract 5 from both sides of the equation! 11 - 5 = 6, so we can write x = 6 We got it! So, we bought 6 video games and 5 movies, and have \$5 left to spare! Good job!

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