First set up a buy-sell-profit equation per orange.
One orange will cost
Selling will get you
(Profit per orange is then
And the gain% will be:
There are short-cuts if you realise that a score = 4x5
And a dozen = 4x3. You can find these out for yourself.
But the above is the way that always works, even if the selling quantity is way different from the buying quantity.
BTW 15 dozen will give a net profit (in RS) of: