super super late ans but im up late n looking 4 interesting Qs...
Potassium-15 is confusing indeed! What neutrons can one add to a nucleus with 19 protons to get a mass of 15? anti-matter cannot do it as scientists have generally agreed that anti-matter has the same mass as regular matter but with an opposite spin.
the ans actually doesnt exist when the Q was originally asked but in 2017, Washington State University physicists created a fluid with negative mass. so theoretically a "negative neutron" will have a mass of -1. combining 4 of such exotic particles to 19 protons will create K-15; assuming there is no explosion from the combination.