One way of doing it is to realize that if she buys 5 bottles, she would need exactly $6. $15.76 that she has includes 2 $6's, which means that she can buy at least 10 bottles. After these 10 bottles, she would have spent $12, therefore having $3.76 left. This is enough for 3 more bottles, costing $3.6 total. 3.76-3.6=$0.16, which is the amount she has left over, after buying a total of 13 bottles.
