Explanation:

Clearly given MP (market price ) will be same for all shops , so lets take market price to be “ x”
Now SP = MP * ( 100 – discount ) / 100
(Or)
SP= x * (100 – discount) / 100
(we all know we sell a product after deducting its discount percentage from the total percentage )
In the given question average selling price is given i.e. 4293 for 3 shops , so the total price of all the three after the discount will be 4293 * 3 = 12879
Writing in equation form,
12879 = 74x/100 + 72x/100 + 66x/100
(SP of A = (100 – discount ) * x / 100 = (100 -26)* x / 100 = 74x/ 100
Similarly for B and C
12879 = 212x100
x = 12879 * 100 / 212
Marked price (x)= 6075 
We found the marked price , next step is to find the selling price of ARTICLE I by shop B
SP = 72x/100
SP= 72 * 6075 / 100
Sp = 4374