Student in a dancing class are spaced evenly around a circle and are then counted consecutively?

from one student to #20 is opposite student #53. How many students are there in the group?

Student in a dancing class are spaced evenly around a circle and are then counted consecutively?
If I start to count from #20 instead of #1 than #20 become #1 (let's call her #1') and #53 becomes #34'.





To make thing clearer if #34' faces #1' than #33' has to face the student before #1' which is labelled #N' where N is the number of students.





It is easy to realize that N has to be even (otherwise all the "facing" part wouldn't make sense), if you think a bit you will also realize that N = 2 * 33 = 66





Here is a simple diagram with only 8 students that should give you a bit of insight:





1 2 3


8 + 4


7 6 5





#1 is facing #5, before #5 there is #4; the total number of students is 8 = 2*4
Reply:my answer is...





72 students...