Riya purchases some pen @5 pen for Rs.4 and she sells all the pen @4…

             Price    :    Qty

Buy :        4      :      5             ......(i)

Sell :       5      :      4            ......(ii)

(i) × 4 & (ii) × 5, to make the equal quantity.

             Price    :     Qty

Buy :      16      :      20

Sell :      25      :      20

Here,

C.P. of 20 articles = Rs.16

S.P. of 20 articles = Rs.25

Profit = 25 - 16= Rs.9

Hence,

Profit percent = \(\frac{9}{16}\) × 100% = 56\(\frac{1}{4}\)%