why is 6/8 greater than 5/8 but less than 7/8
Because 6/8 means, dividing 6 by 8. If you divide 7 by 8 eight you will get a greater number then 6/8 because 7 is greater than 6. And if you divide 5 by 8 you will get a smaller number than 6/8 because 5 is less than 6. I hope its not confusing.
6 comes before 7 and after 5, so 6 would make the number 5 less than it and 7 would make it greater than 6