I sell cookies for $3 and muffins for $7. Yesterday, I made $129. I sold a total of 27 items. How many of each did I sell?

You sold 15 cookies and 12 muffins

Step-by-step explanation:

So you're given that 1 cookie is sold for $3 and 1 muffin is sold for $7

You're also given that in total you made $129, and sold 27 items

Let's make equations from that, letting c stand for amount of cookies and m for amount of muffins.

Now we can use the second equation to find m:

c + m = 27

m = 27 - c

Not we can substitute m into the first equation to find c:

3c + 7m = 129                  

3c + 7(27-c) = 129            [substitute m for (27-c)]

3c + 189 -7c = 129           [distribute 7 into (27-c)]

-4c + 189 = 129                [combine like terms]

-4c = -60                          [subtract 189 from both sides]

c = 15                               [divide both sides by -4]

Now we can substitute c into the second equation to find m:

c + m = 27

15 + m = 27

m = 12