Answer
15
Work Step by Step
Letting
A= set of models of the iMac,
B= set of colors,
a set containing all combinations could be modeled as
$\mathrm{A}\times B=\{(a, b)|a\in A$ and $b\in B\}$\\
(the set of all ordered pairs whose first component is in $A$,\\
and whose second component is in $B$.)
The cardinality of $\mathrm{A}\times B$ is
$n(A)\cdot n(B)=3\cdot 5=15$