Sonia purchases some pen @11 pen for Rs.12 and she sells all the pen @12 pen for Rs.11. Find overall profit or loss %.

17.4% 

             Price    :    Qty

Buy :        12      :     11             ......(i)

Sell :        11       :      12             ......(ii)

(i) × 12 & (ii) × 11, to make the equal quantity.

             Price    :     Qty

Buy :      144      :      132

Sell :      121      :      132

Here,

C.P. of 132 articles = Rs.144

S.P. of 132 articles = Rs.121

Loss = 144 - 121 = Rs.23

Hence,

Loss percent = \(\frac{23}{132}\) × 100% = 17.4%