By selling 90 ball pens for Rs 160 a person loses 20%. How many ball pens should be sold for rs 96 so as to have profit of 20%?
Selling price of 90 ball pens = Rs. 160
Selling price of 1 ball pen = Rs. 
Loss% = 20%
Let cost price of 1 pen = Rs. X
So, 
= 
Cost price of 1 ball pen = Rs.
To get profit of 20%...
Let no.of pens = x
Selling price of x pens = Rs. 96
Selling price of 1 pen = Rs.
By formula ,
= 
= 20 = 
= 
= x = 36
Hence,
Number of ball pens can be purchased = 36.
15