Timothy explains to his brother that the model y=4mt has a constant of variation of 4 and is an example of variation.

Joint variation.

Step-by-step explanation:

There are four types of variation,

Direct variation : When a variable varies directly as another variable,

Example : y varies directly with x ⇒ y = kx,

Where, k is the constant of variation,

Inverse variation : When a variable varies inversely as another variable,

Example : y varies inversely with x ⇒

Joint variation : When a variable varies directly with two or more than two variables,

Example : y varies directly with both x and z ⇒ y = kxz

Combined variation : When a variable varies both directly and indirectly as another variables,

Example : y varies directly with x and inversely with z ⇒

Now, here the given expression,

y = 4mt,

Where, y, m and t are variables and 4 is the constant of variation,

⇒ y varies directly as both m and t,

⇒ y = 4mt is an example of joint variation.