For this challenge, I used the equation 0(Current fuel) = 100(max fuel) -(B * 120(Time elapsed)).
After isolating the variable B, I determined that the burn rate for a ship with 100 fuel to make it 120 is .8333/second.
We could also adjust the max fuel. In order to do that, our equation would be M(max fuel) = 0(current fuel) + (120 * 2). After solving, in order to use our current burn rate of 2 our max fuel would need to 240 units.