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Answer

S : W= 4 : 1 ,, means , 

20kg(Spirit) : 5kg(water)  #starting 

33.33% water means 1/3 part water = 2part(spirit) : 1part(water) #required

we are not adding any spirit so in final mixture spirit will be 20 kg itself ,so we basically need , 20kg(spirit) : 10kg(water)

here extra 5kg of water is required as 5 kg is starting amount of water we need only 5kg to make it 10kg water, so answer will be 5kg