30 * 5 - 10n ≥ 100
For this challenge, I first multiplied 30 * 5. Then the equation became 150 - 10n ≥ 100.
After that, I subtracted 150 from each side and the equation became -10n ≥ -50.
After that, I divided each side by -10. This forced me to flip the sign and led to n ≤ 5.
So, the player could refuel no more than 5 times in order to still be able to upgrade their ship at the end of the level.