Fraction Investigation

Take four numbers say 2, 5, 7, 15. Use the four numbers to make two fractions, for example: 2/5 and 7/15. There are 12 ways you could have done that. For the numbers a, b, c, and d, the value a could go with the b, c, or d. That's 3 choices. The un-chosen letters then make the other pair without any further choice. For each of the two pairs there are 2 possibilities, the numbers could be either way up: a/b or b/a. So 3 . 2 . 2 = 12 arrangements. Which of these arrangements gives the greatest (or least) sum (or difference)? That's four questions.