Given,
z is directly proportional to the product of x and y.
The value of z is 15 when x is 5 an y is 6.
According to the question,

Here, k is proportionality constant.
Substituting the value of x, y and z then,

Susbtituting the value of k in the above expression then,

Hence, the equation that shows the relation of x, y and z is z=1/2xy.