Question

Roshan Bought 150 hens for Rs.1200 and sold 20% of these at gain of 5% at what gain percent must he sell the remaining hence so as to gain 20% in whole transaction?

Answer 1

He bought 150 hens for 1200,i.e. each for rs8
by condition, he sold 20% of his hen  i.e. 30 costing 240 at the gain of 5%
means 240+12 = rs 252
Now remaining hen = 120 
target gain is 20% on 1200 means 1440
now he has to sell 120 hen in order to earn 1440 in total
252 he has already earned so now he'll have to sell 120 hen for 1188

Answer 2

20% gain in whole,
gain = 20% of 1200
        =(20/100) ×1200
        =240
SP = 1200 +240 = 1440
CP of 150 hens = 1200
CP of 1 hen = 1200/150 =8
CP of 30 hens = 30×8 = 240
5% of 240 =12
20% hens is sold in 5% gain
means 30 hens sold in (240+12) =252
now. he should the 150 hens in 1440
so, SP of rest 80% hens = (1440- 252) = Rs 1180