Varsity Tutors
Varsity Tutors: Hotmath: Practice Problems: Variations
Twenty-three problems cover a variety of topics applied to variations. They are given with each step to the solution cleverly revealed one at a time. You can work each step of the problem then click the "View Solution" button to check...
Wolfram Research
Wolfram Math World: Direct Proportion
MathWorld offers a concise mathematical definition of direct variation or direct proportion.