Literally plugging in the time for each value, we can plug in 5 for t to get
22000+(-480*5)=22000+(-2400)=22000-2400=19800
Plugging in 10, we get 22000+(-480*10)=22000-4800=17200. We can turn 22000+(-480*10) into 22000-(480*10) using the distributive property, and adding a negative number to something essentially means to subtract that number.
I challenge you to do t=15 and 20 on your own!
