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.