Nikki wants to buy a dog bed that costs $87. she earns $6 per week for doing chores.

I think in this question you want to know the number of days it will take Nikki to buy the dog bed. Based on this assumption i am proceeding with the problem. There are several information's given in the problem and it is required to note them down first.
Cost of the dog bed = $87
Amount Nikki earns each day doing chores = $6
Let us assume the number of days Nikki requires to collect this amount = x
Then
6x = 87
x = 87/6
   = 14.5 days
As it cannot be 14.5 days so it should be 15 days. there will be some excess amount in the hands of Nikki. If we take 14 days then the amount collected by Nikki will be less.So it will require Nikki to work for 15 days to collect the amount needed for buying the dog bed.