What is a direct variation?

A direct variation is when the value of K in multiple proportions is all divisible by the same number for example:

XY=(1)(10) K=10

XY=(2)(20) K=40

XY=(3)(30) K=90

XY=(4)(40) K=160

In this situation the constant (K) of each proportion is divisible by 10 making the multiple equations a direct variation.