Answer:
[tex]160\ \text{s}[/tex]
Step-by-step explanation:
The inequality that represents the situation is
[tex]500-2.5t<100[/tex] where [tex]t[/tex] is the time in seconds
[tex]\Rightarrow -2.5t<100-500[/tex]
[tex]\Rightarrow -2.5t<-400[/tex]
[tex]\Rightarrow 2.5t>400[/tex]
[tex]\Rightarrow t>\dfrac{400}{2.5}[/tex]
[tex]\Rightarrow t>160\ \text{s}[/tex]
So, the time after which there will be less than 100 gallons of water is [tex]160\ \text{s}[/tex].