Question

a farmer bought 100 pens for rupees 4000 and sold 20 of them at a gain of 5% at what gain percent must he sell the remaining so as to gain 20% on the whole

Answer 1

CP of 1 article = 4000/100 = Rs 40
He sells 20 articles at a gain of 5%
So,
Selling Price Of Those 20 Articles = 40*20 + 40*20*5/100
                                                       =800 + 40
                                                       = Rs 840

On the whole he wants 20% profit 
which means the profit he wants = 20/100   * 4000
                                                     = Rs 800

CP of 80 Articles = 80*40 = Rs 3200
From Rs 800 he already got Profit Of Rs 40 Which Means he has to get More Rs 760

P% = P/CP * 100
      = (760 / 3200) * 100
      = 23.75%