An alloy contains 40% of silver, 30% of copper and 30% of nickel. How much silver (in kg) should be added to 25 kg of the alloy so that the new alloy contains 50% of silver?

5

Total quantity of alloy = 25 kg,

Ratio

Silver : Copper : Nickel

  4     :    3       :    3

⇒ (4 + 3 + 3)x = 25,

⇒ 10 x = 25,

⇒ x = 2.5,

Quantity 

                    Silver   :     Copper     :    Nickel

                4 x 2.5     :    3 x 2.5      :    3 x 2.5

 Quantity         10     :      7.5          :       7.5

⇒ To make silver 50 %,

⇒ Silver = Copper + Nickel

⇒ 10 + x = 7.5 + 7.5

⇒ x = 15 - 10

⇒ x = 5 Kg

Hence 5 Kg silver should be added to alloy.