Explanation:
"t t hours" ????
you mean 11 hours ?
13 gallons now.
2 gallons are used (and therefore disappear from the tank) per hour.
so, in 5 hours 5×2 = 10 gallons will be used.
that leaves 13 - 10 = 3 gallons in the tank.
in 11 hours 11×2 = 22 gallons would have been used.
but there are only 13 gallons in the tank.
so, if he does not fill up, he will have 0 gallons left after 11 hours. in fact, already after 13/2 = 6.5 hours.
the point :
when you use 2 gallons per hour, that means in t hours you are using t × 2 or 2t gallons.