The exponential dilemma might be a good bet.

lets say that there are 120 gallons per tank.

lets say that we can fill twice the gallons to the tank in the same time that we did before, a minute. So that in the first minute we fill one gallon.

1th minute 1 gallon       **1**
2nd minute 2 gallons      **2**
3rd minute 4 gallons      **4**
4th minute 8 gallons   +  **8**
5th minute 16 gallons     **16**
6th minute 32 gallons     **32**
7th minute 64 gallons     **64** = 127 total gallons, a little more  
8th minute 128 gallons              than than one tank.
9th minute 256 gallons

At this rate it took 7 minutes to fill a 120 gallon tank, the next minute will fill a whole new tank, and the next minute will fill 2 tanks

To summarize:

at the 7th minute we have a filled tank at the 8th minute we have a new filled tank at the 9th minute we have two more new tanks

Total of 4 tanks in 9 minutes, one in 7 minutes, one in 1 minute and two more in 1 minute.

this rate can be a a solution for your problem.

I can do a calculation similar to this which is going to take less time than 7.5 billion years but I will do it later because I can't now.

The exponential dilemma might be a good bet.

lets say that there are 120 gallons per tank.

lets say that we can fill twice the gallons to the tank in the same time that we did before, a minute. So that in the first minute we fill one gallon.

1th minute 1 gallon       **1**
2nd minute 2 gallons      **2**
3rd minute 4 gallons      **4**
4th minute 8 gallons   +  **8**
5th minute 16 gallons     **16**
6th minute 32 gallons     **32**
7th minute 64 gallons     **64** = 127 total gallons, a little more  
8th minute 128 gallons              than than one tank.
9th minute 256 gallons

At this rate it took 7 minutes to fill a 120 gallon tank, the next minute will fill a whole new tank, and the next minute will fill 2 tanks

To summarize:

at the 7th minute we have a filled tank at the 8th minute we have a new filled tank at the 9th minute we have two more new tanks

Total of 4 tanks in 9 minutes, one in 7 minutes, one in 1 minute and two more in 1 minute.

this rate can be a a solution for your problem.

EDIT

2^33 = 8589934592

you can say the sun death was predicted in linear growth but that it actually is in exponential growth. Using the example from above changing a minute for a year, it will take 33 years.