Let 1kg of Rs. 100 then 840gm is of Rs. 84.
Now (label on can 1kg but contains 840kg ) so for customer it is of Rs. 100 and further gives 4% discount [he sells his article on 4% loss on cost price.]
So now S.P = Rs. 96
But actually it contains 840 gm so C.P for shopkeeper = Rs. 84
S.P = Rs. 96
C.P = Rs. 84
Profit% = {(S.P-C.P)/C.P}x100
{(96-84)/84} x 100 = 14.28% PROFIT.