A shopkeeper purchased 50 dozen cups for Rs. 3000. Out of these, 60 cups were broken and could not be sold. At what rate per dozen should he sell the remaining cup to make a profit of 20% ?

Cost Price = Rs. 3000
To make a profit of 20%, selling price should be,
$$\eqalign{ & {\text{SP}} = 3000 \times \frac{6}{5} \cr & \,\,\,\,\,\,\,\,\, = 3600 \cr} $$
As 5 dozens cup were broken out of 50 dozen
So, selling price of remaining 45 dozen cup :
$$ = \frac{{3600}}{{45}} = {\text{Rs}}{\text{. 80}}$$