Andrew has a collection of soda bottles. some of them are 12 ounce bottles, and others are 16 ounce bottles. if the collection contains 20 bottles, which hold a combined 300 ounces, how many of the soda bottles are 12-ounce bottles?

Use a for the 12 ounce bottles and b for the 16 ounce bottles. Now make two equations, one to show total ounces and one total bottles.
12a + 16b = 300 (12 ounces in a number of bottles plus 16 ounces in b number of bottles it 300 total bottles).
a + b = 20 (total number of bottles.)
Now use the substitution method to solve for a, the number of 12 ounce bottles:
a + b = 20
- a       - a
b = 20 - a (substitute 20 - a into the other equation for b)
12a + 16(20 - a) = 300  (Use distributive method
12a + 320 - 16a = 300  (Combine a's and subtrace 320)
       - 320          - 320
-4a = -20   (Divide by -4)
a = 5
There are 5 12 ounce bottles.