Which of the following types of models is usually in the form of a drawing graph or equation

The type of model that is usually in the form of a drawing, graph, or equation would be an idea model. 

computer models are usually collections of code, physical models are usually actual physically created models that have their own characteristics. An idea model, on the other hand, is a type of model that can be explained either through a drawing, a graph, or an equation

Fossil found in strata reveal that unused body part decrease in size