Finally, it is important to address problems that have feasible solutions. The more tractable the solution to a problem is, the greater the likelihood of actually solving the problem. This means it is preferable to work on a problem that has a number of possible solutions, or at least a number of possibilities for improving the situation.
An example: Imagine your goal is to save and/or significantly improve lives. In that case, funding cancer research is not an effective use of your resources. All the research into metastatic cancer over the last 10 years has only resulted in a life extension of one month. One research year per scientist costs an average of 50,000 euros. For that same amount of money you can provide 20,000 doses of vitamin A, preventing blindness in children and saving 14 lives.