Skepticism about saving the greater number

Suppose that each of the following four conditions obtains: 1. You can save either a greater or a lesser number of innocent people from (equally) serious harm. 2. You can do so at trivial cost to yourself. 3. If you act to save, then the harm you prevent is harm that would not have been prevented if you had done nothing. 4. All other things are equal. A skeptic about saving the greater number rejects the common-sensical claim that you have a duty to save the greater number in such circumstances.