You buy bottled water in 12- packs that cost $3 each. if u drink 3 bottles per day , what is your cost per week?

Cost of bottles that come in packs of 12 = 3 dollars
It is already given in the question that i drink 3 bottles per day. So the cost of 3 bottles has to be determined first and then the number of bottles that is drunk in a week also has to be determined. after that the cost per week can be easily determined.
Number of bottles drunk per day = 3
So
The number of bottles drunk in a week = (3 * 7) bottles
                                                               = 21
Now 
Cost of a pack of 12 bottles = 3 dollars
Then
Cost of 1 bottle = (3/12) dollars
                         = (1/4) dollars
                         = 0.25 dollars
So
The cost of 21 bottles = (21 * 0.25) dollars
                                    = 5.25 dollars
 So the cost per week as determined above is $5.25.