x gallon of 1% grade milk will have x/100 gallons of pure milk
y gallons of 2% grade milk will have 2y/100 gallons of pure milk
z gallons of 3% grade milk will have 3y/100 gallons of pure milk
(here by pure I mean 100% fat milk)

After mixing we get x+y+z gallons of 1.5% grade milk i.e 3(x+y+z)/200 gallons of pure milk

Equating the equations:
x/100 + 2y/100 + 3z/100 = 3(x+y+z)/200

Solving this we get x = (y+3z)

SanjeevK,

Can you elaborate this please:
1.5% grade milk i.e 3(x+y+z)/200 gallons of pure milk

Thanks

convert 1.5% to fraction. 1.5% = 1.5/100 = 3/200
Hope this helps

Many thks