Question

A shopkeeper buys 100 pens for Rs.200 and sell 20 of them at a gain of 10%.at what gain percent must he sell the reminder so as to gain 20% in all?

Answer 1

for 20 pens
cost pries of 20 pens = 200*20/100 = Rs.40
gain% = gain*100/CP
10 = gain*100/40
gain = Rs.4
for 100 pens
gain% = gain*100/CP
20 = gain*100/200
gain = Rs.40
let he sell remaning 80 pens at x% gain
cost pries of 80 pens = 200*80/100 = Rs.160
the gain should be 40-4 = Rs.36 (according to question)
36 = x*100/160
x = 57.6%
hence gain% = 57.6

Answer 2

ans. 22.5%

he buy 100 pen of rs 200/- & now he want sell on 20% profit.
 so his total sell =200 x 20%=40/- = 200+40=240/-.

he sell first 20 pen on 10% gain
there per pen cost 200/100= 2/- Per pen

so 20pen sell at 20 x 2=40/- and 40 x 10%=4/-
now we have 40+4=44/-

less 20pen costing to total sell
240-44=196/-

he have 80 pen left. its buying cost
80*2=160/-

less 80pen costing to balance sell costing 
196-160=36/-


if he make a profit of rs. 36/- on 80pen then he get the 20%gain on all sell.

36/160/100=22.5 %