Direct Inverse Joint and Combined Variation

The equation 
xy =  25  represents:

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 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 grade you earn in a class
and the number of assignments you complete
and the number of hours you study

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.