$28 saved in 4 weeks = dollars per week

In 4 weeks the amount of money saved = $28
Then
In 1 week the amount of money saved = (28/4) * 1 dollars
                                                               = 7 dollars.
So
$28 saved in 4 weeks = 7 dollars per week.
I hope the procedure is easy enough for you to understand. You can always use this procedure to get to the solution of similar problems in future without requiring any help from outside. Normally this procedure can be used to determine the amount for any number of weeks or days. Only thing is if the amount in days are required, then the weeks have to be converted to days before calculating.