On a recent trip to the convenience store, you picked up 3 gallons of milk, 4 bottles of water, and 8 snack-size bags of chips. your total bill (before tax) was $21.30. if a bottle of water costs twice as much as a bag of chips, and a gallon of milk costs $1.60 more than a bottle of water, how much does each item cost?

To find the answer you will write and equation in terms of the cost of a bag of chips.  

3(cost of milk) + 4 (cost of water) + 8 (cost of bag of chips) = $21.30
   3 (2c +1.60)  +        4 (2c) +        8 (c) = $21.30 ; Simplify
  6c + 4.80   +  8c   + 8c = 21.30
    22c + 4.80 = 21.30
           - 4.80     -4.80
  22c      = $16.50
  22               22
c = 0.75

The chips cost $0.75 each.

The milk costs (2 x $0.75 + $1.60) $3.10 each.

The water costs (2 x $0.75) $1.50 each.

The answer is 6 weeks