You have a situation, the exact parameters of which do not matter.
You have a choice to make.
Choice A will CERTAINLY kill 15 people, and has the POTENTIAL to kill *UP TO* an additional 90 people.
Choice B will CERTAINLY kill 20 people, and has the POTENTIAL to kill *UP TO* an additional 75 people.
You do not know probabilities for these potentials; it could be only 1 or 2, it could be 65 or 70, it could be the full respective 90 and 75.
What do you do, and why??