Seven bottles of soda cost $21.00 while 4 bottles of water cost $8. If David wants to buy 3 bottles of soda and 2 bottles of water, how much will that cost?
Find the solution. 
One bottle of soda costs $21.00/7 = $3.00.
Three bottles of soda cost $3.00 x 3 = $9.00.
One bottle of water costs $8.00/4 = $2.00.
Two bottles of water cost $2.00x 2 = $4.00.
Therefore, 3 bottles of soda and 2 bottles of water cost $9.00 + $4.00 = $13.00.