
Hi
the trick is to define price by referring only to one item. Â
Here the bootle of water is you reference. Â
there is 3 gallons  of milk
       6 bottles of water
       6 bags of chips
let's call bottle of water  " X" Â
water cost  = X
milk = X+1.7
chips = Â X/2
So we have  : Â
3 (X+1.7 ) + 6X + 6 ( X/2) = 29.1
3X+ 5.1 + 6X + 6X/2 = 29.1
3X +6X + 3X + 5.1 = 29.1
 12X +5.1 = 29.1
 12X = 29.1 - 5.1
  12 X =  24
    X = 24/12
    X = 2
price of bottle is  2 dollars Â
then milk is  2 + 1.7 = 3.7
    chips =  2/2 = 1
let's check = Â 3*3.7 + 6*2 + 6*1 = Â 11.1+12+ 6 = 29.1