question: A mother goes shopping. She buys cocoa at $4.20, laundry at $9.45 and a package of pasta at $1.35. She pays $20. How much change does the cashier give back?
answer: The total shopping cost is $4.2 + $9.45 + $1.35 = $<<4.2+9.45+1.35=15>>15.
  The cashier gives back $20 - 1$5 = $<<20-15=5>>5.
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