A dealer had 120 kg of wheat. A part of it was sold by him at 10% gain and the rest at 25% gain. overall, he had a gain of 15%. How much of the wheat was sold at 10% gain?

80 kg

      Using allegation,

      10%                                 25%

                            15%

      10                   :                  5

       2                   :                  1

      ⇓ × 40                            ⇓ × 40

      80                                      40

Sold at 10% gain = 80 kg.