How many kilograms of butter of rs 140 per kg must be mixed with 20 kg of butter worth rs 150 per kg to produce a mixture worth rs 144 per kg. explain this

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 30

i pretty sure it is

answer: c

(2, -4)

goes 7 to the

(9, 4)

easy