Varies Directly

$\mbox{If }y \mbox{ varies directly with }x \mbox{ then }y = c x \\ \mbox{If }y \mbox{ varies indirectly with }x \mbox{ then }y = \dfrac c x \\ \mbox{For some number }c$

An example of direct variance.

The number of teachers in a school varies directly with the numer of students.

Because, if there are more students then there will be more teachers.

Say there is 1 teacher to 25 stuents 

then      

$T=\frac{1}{25}\;S\\ or\\ T=\frac{S}{25}$