Answer:
We know that the prize of 6 bags of marbles is $18.
Then if each one of the bags costs C, the 6 bags will cost a total of 6 times C.
Then the cost is:
6*C
and this is equal to $18
6*C = $18
Now we want to solve this for C, remember that we can divide both sides of the equation by the same number, then let's divide both sides by 6:
6*C/6 = $18/6
C = $18/6 = $3
Each bag costs $3.
Below you can see a double number line, where above is the number of bags and below is the prize:
