10 cookies and 3 brownies cost $11.25 6 cookies and 9 brownies cost $15.75 what is the cost of one brownie?

One brownie costs $1.25

Step-by-step 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