Question 308306
Ziggy bought x cans of red goo for y dollars a can
Total cost of red goo = xy
Also,Ziggy bought z cans of blue goo for 3y dollars a can. 
Total cost of blue goo = z*3y=3yz
He bought twice as much blue goo as red goo.
So, z=2x
Total number of cans = z+x= 2x+x = 3x
Total cost of all goo = xy+3yz=xy+3y*2x=xy+6xy=7xy
Average cost per can = (7xy)/(x+z)=7xy/(x+2x)=7xy/3x=$ 7y/3