The correct answer is that there will be four runners needed. We can work this out in the following way - if every runner runs a sixteenth of a mile, then you need to work out how many sixteenths are in a fourth. You can establish this by converting the fraction so it becomes equivalent. 1/4 can be multipled by 4 to equal 4/16th, so therefore if there are four 16ths in one 1/16th, you will need four runners.