The total monthly income, $D, of a salesman who sells tyres consists of a basic salary of $600 and
a variable amount which depends on the
number of tyres he sells. For each tyre he sells,
he receives $8.
(i) If the salesman sells 95 tyres in a particular
month, find his total income for that month.
(ii) If the salesman's monthly income for a particular
month is $1680, calculate the number of
tyres he sells in that month.
(iii) Write down a formula connecting D and n,
where n is the number of tyres the salesman
sells in a month.
(iv) Draw the graph of D against n. Is D directly
proportional to n? Use your graph to explain
your answer.​