hi tissuebox,
let x= the number of bottles the customers buys
first, lets consider how many bottles the customer must buy for the price to be the same no matter which company.
4x+10=3x+25
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x=15, so at 15 bottles, the cost is the same for either company. At this point you look at x=16, one bottle higher, to compare costs, seeing that for company A this is $74 while for company B this is $73, so at 16 bottles company B is cheaper