question: 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?
answer: One bottle of soda costs $21.00/7 = $<<21/7=3.00>>3.00.
  Three bottles of soda cost $3.00 x 3 = $<<3*3=9.00>>9.00.
  One bottle of water costs $8.00/4 = $<<8/4=2.00>>2.00.
  Two bottles of water cost $2.00x 2 = $<<2*2=4.00>>4.00.
  Therefore, 3 bottles of soda and 2 bottles of water cost $9.00 + $4.00 = $13.00.
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