What is the equation for direct variation?

What are the equations for inverse variation? Choose 2.

What type of variation does this scenario represent?
You and some friends decide to buy 2 pizzas and split the cost evenly.
The number of friends who are sharing the cost of the pizzas
and the amount of money you pay for your share.

What type of variation does this scenario represent?
The outside temperature
and the number of layers of clothes you need to wear outside to feel comfortable 

What type of variation does this scenario represent?
The length of time you charge your cell phone
and the battery power 

What type of variation does this scenario represent?
The less you eat junk food
and the affect on your weight.