If you are working with time or days, you can't put in a negative value because if you are doing an experiment, you can't go back in time to do that experiment to gather data. (Time for the most part considered to be an input value so that's why I used it as an example.)
Let s equal the number of shirts she can buy
140 > $28.50 + $20.75s
111.50 > 20.75s
5.37 > s
so we round down. she can buy 5 shirts, plus the one dress and she would spend less than $140
One and a half = 1.5
Three and a half = 3.5
1.5 + 3.5 = 5.
