It can't be joint, for there has to be more than two variables
to have joint variation.

To tell whether it is direct or inverse:

Substitute any two different values for X and calculate the two 
different values of Y for each.

If when X INcreases, Y also INcreases, and
when X DEcreases, Y also DEcreases then the variation is DIRECT.
(they go up or down in the SAME direction),

If when X INcreases, Y DEcreases, and
when X DEcreases, Y INcreases then the variation is INVERSE.
(they go up and down in OPPOSITE directions),



Substitute Y = 3:







Substitute Y = 6:


 
  




So when X INcreased from 3 to 6, Y also INcreased
from 1 to 2, so the variation is DIRECT. 
(They both went UP).

----------------------- 



Substitute x = 2:





Substitute x = 5:


 


So when X INcreased from 2 to 5, Y DEcreased
from 5 to 2, so the variation is INVERSE.
(One went up and the other went down).

Edwin