A milkman sells 120 liters of milk for rs. 3360 ans he sells 240 liters of milk for rs. 6120.how much concession does the trader give per liter of milk.when he sells 240 liters of milk?

The smaller quantity is sold for (rs 3360)/(120 L) = rs 28/L.
The larger quantity is sold for (rs 6120)/(240 L) = rs 25.5/L.

The latter price represents a price adjustment of
  (25.5/28 - 1)*100% ≈ -8.93%

The concession is about 8.93% on the per liter price for the higher volume.
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The actual value is 8 13/14%.