it seems there are not fractions between 3/5 and 4/5, but there are so many
for example, if you make denominator to 10 for both fractions, you will get 6/10 and 8/10
you can say 7/10 is in between 3/5 and 4/5
also, if you make denominator to 100 for both fractions, you will get 60/100 and 80/100
you can get 20 fractions between 3/5 and 4/5