Cost price of oranges are given as Rs.26/-per dozen
Selling price of 5 oranges =Rs.13
Therefore, selling price of oranges= 13/5
Seling price of 12 oranges =135×12=Rs.31.20 per dozen
Selling price of oranges >cost price of oranges.
So, gain occurs.
Gain on selling a dozen of oranges =S.P−C.P.=31.20−26=5.20
Gain% =Gain×100=5.2×100=52×100=20 %