.
The volume of the leakage is the integral of the function over the time from t= 0 to t= 90 minutes.
This integral is equal to = = = 60*(1-0.000123) = 60 liters
(with the accuracy absolutely enough for any engineering purposes).
By knowing the leakage volume, you calculate the amount of fuel left in the reservoir as the difference
1200 liters - 60 liters = 1140 liters. ANSWER
Notice that starting leakage rate was 6 liters per minute.
At the time t= 90 minutes, the leakage rate is = = 0.00074 liters per minute.
So, the change of the leakage rate during 90 minutes is very significant.