Answer
a. $5$
b. $3$
Work Step by Step
$$n=dq+r=d(n\ div\ d)+n\ mod\ d$$
$$28=5(28\ div\ 5)+28\ mod\ 5=5(5)+3$$
The numbers $n\ div\ d$ and $n\ mod\ d$ are defined to be the numbers $q$ and $r$, respectively, as given in the quotient-remainder theorem. See page 181 of the text.