Greetings!
Answer:
One brownie costs $1.25
Explanation:
We need to solve this with simultaneous equations. First, write the equations out in two lines:
c = cookies and b = brownies.
10c + 3b = 11.25
6c + 9b = 15.75
We need to make either the two c values the same or the two b values the same. Lets change the two b values to the same by multiplying the first equation by three:
10c + 3b = 11.25
10c * 3 = 30c
3b * 3 = 9b
11.25 * 3 = 33.75
30c + 9b = 33.75
Now we can subtract the second equation from this:
30c - 6c = 24c
9b - 9b = 0
33.75 - 15.75 = 18
24c = 18
Now divide both sides by c to find 1c:
c = 0.75
Now put this back into the first equation:
10(0.75) + 3b = 11.25
7.50 + 3b = 11.25
Move the +7.50 over to the other side making it a negative:
3b = 11.25 - 7.50
3b = 3.75
Divide both sides by 3:
b = 1.25
So one brownie costs $1.25
Hope this helps!