A shopkeeper bought certain number of
pens at the rate of 20 Rs60 and the same
number at 30 for Rs40. After that he
realized that he had bought only 60% of
the required number of pens. He bought
the rest at 25 for Rs75. He sold the pens at 12 for Rs35. Find his profit/loss %?

Assume he bough 60 pens each for both the Price (60 and 40)
Then you'll have 120 pends with C.P as 240
But 120 pens are just 60% of actually required
-> You require 200 pens so you buy 80 pens (25 for Rs75 i.e. Rs3/each)
So total pens 200 with C.P = 500
C.P per pen = 2.5
S.P is 35/12
Calculate profit. Approximately I can tell it will be less than 20%