His entire purchase cost $20 - $4 = $16. The chocolate bars cost 5 * $2 = $10. The bags of chips cost a total of $16 - $10 = $6. Each bag of chips cost $6 / 2 = $3.
This justifies what answer for what question? Q & A:
Frank goes to the store to buy some food. He buys 5 chocolate bars and 2 bags of chips. He hands the cashier $20 and gets $4 back as change. If the chocolate bars each cost $2, how much did each bag of chips cost?
3