Here we have two set, the Domain is the number of items she buys and the Range is the amount of money she spend each month.
Between this two set there is a relationship, because the clothes have a price.
But also we have a condition, a constraint, in the statement. Margaret has a budget of $50 per month. This constraint limit the numbers of items she can buys each month, and the Range too.
The constraint in the Domain is the cost of the number of items could not be more than $50 each month.
