A mixture of 20 kg of water and spirit contains 10% water. How much water must be added to this mixture to raise the percentage of water to 25%

Acc to question this mixture contain 18kg spirit and 2kg water as per the question, we have to add water to make the concentration equal to 25% so the amount of spirit will remain unchanged in the final mixture spirit = 18 kg which represent 75% of the final mixture (as remining 25% is water) applying formula for percentage 75%=18kg then 100% would be (18*100)/75 =24 which means we have to add 4kg of water.

2 units will be added 1 unit is 2kg therefore ans is 4kg