one question from GMATprep

three grades of millk are 1 percent, 2 percent, and 3 precent fat by volume. If X gallons of the 1 percent grade, Y gallons of the 2 percent grade, and Z gallons of the 3 percent grade are mixed to give X+Y+Z gallons of a 1.5 percent grade, which is X in terms of Y and Z?

(1) Y+3Z  ( this one is right)

(2) (Y+Z)/4

(3) 2Y+3Z

(4) 3Y+Z

(5) 3Y+4.5Z

thanks for help..really don't know how to do this one..

(x*1%+y*2%+z*3%)/(x+y+z)=1.5%  x=Y+3Z 

A