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

the multiples of 15 are 15, 30,45, 60, 75, 90, 105, 120, 135, 150, 165, 180, 195,   210, 225, 240, 255, 270, 285, 300, 315, 330, 345, 360, 375,   390, 405, 420, 435, 450, 465, 480, 495, 510, 525, 540, 555, 570, 585, 600, 615, 630, 645, 660, 675, 690, 705, 720, and 735.

the factors of 15 are 1, 3, 5, and 15.

hope that , fam!

-une, your fren : )