Question

How many kilograms of butter at Rs140kg must be mixed with 20kg of butter worth Rs150kg to produce a mixture of 144kg

Answer 1

let x kg of butter @Rs140/kg be added to 20kg of butter @Rs 150/kg
total price = 140×x + 150×20 = 140x + 3000

after mixing, (x+20) kg of butter is produced @Rs144/kg 
144×(x+20) = 140x+3000
⇒144x + 144×20 = 140x + 3000
⇒ 144x - 140x = 3000 - 144×20
⇒ 4x = 3000-2880 = 120
⇒ x = 120/4 = 30kg

So required answer is 30kg

Answer 2

Let the Qty of butter of Rs 140 be mixed= x kg
Cost of 20 kg butter= 140*x Rs
Total qty of butter= (x + 20) Kg
 Cost of ( x+ 20)kg  butter= (x+20)*144
ATQ        150*20 + x*140= (x+20)* 144
             ⇒  4x= 120
                     x=30
30 Kg of butter of Rs 140 is mixed.