Answer:
Revenue = $$[n_s - \frac{n_{s}}{k}].S$$
where $n_s$ => No of units ready for sale. $\frac{n_{s}}{k}$ is the free unit to be given away
Cost = $$n_{p}.P$$
We know now that $$n_s = n_p + \frac{n_{p}}{3}$$
$$\frac{[n_s - \frac{n_{s}}{k}] - n_p}{n_p} = .1$$
$$[n_s - \frac{n_{s}}{k}] = 1.1n_p$$
$$n_s.[1-\frac{1}{k}] = 1.1n_p$$
$$n_p.[1+\frac{1}{3}].[1-\frac{1}{k}] = 1.1n_p$$
$$[1-\frac{1}{k}] = \frac{3.3}{4}$$
$$\frac{1}{k} = .175$$
Now for one unit to be given free, it means that
$$\frac{n_s}{k} = 1$$
$$n_s = 5.71 = 6$$ Units to be sold
To use a little basic modular arithmetic ($n_s=40$) and you give away $7$ units and
$n_p =30$ resulting in a profit of $10 percent$. So for every $3$ units purchased you get an additional unit making it $40$ that are now available for sale and to make a profit of $10
percent$ you can choose to give away $7$ units.
Thanks
Satish