Put them into fractions with a bigger denominator (eg:10). You do this by multiplying the top and bottom numbers of both fractions by 2 (10 divided by 5 equals 2). So you now have 6/10 and 8/10 the number in the middle of these number would be 7/10. Therefore, 7/10 is inbetween 3/5 and 4/5. Hope this helps :)