At a store , 24 bottles of water and 6 bags of chips costs $23.40. five bottles of water and two bags of chips cost $6.60. what is the cost of a single bottle of water ?

24w+6c=$23.40

5w+2c=$6.60

So you have to find c (chips) first. You have to make the number of chips in both equations cancel each other.

-3(5w+2c)=-3($6.60)
-15w-6c=$-19.80

Then subtract to get one formula
24w + 6c = $23.40
-15w - 6c = $-19.80

9w = $3.60
w=$.40

To check your answer, find how much one bag of chips costs:
5($.40) + 2c = $6.60
$2.00 + 2c = $6.60
2c = $4.60
c=$2.30

Plug your value for one water and one bag of chips into the formula:
5($.40) + 2($2.30)
$2.00 + $4.60
=$6.60

Then plug your values into the original problem:
24($.40) + 6($2.30)
$9.60 + $13.80
=$23.40