You work at a credit card company and as a value-add they want to start providing alerts to users when their spending in any particular category is higher than usual.
Each Payment comes with a price, description, and category. A Category represents a collection of payments (e. g. "entertainment", "restaurants", "rent", ...).
This is what the new service should do:
For a given user, you have to compare the total amount paid for the last two months, grouped by category; and filter down to the categories for which the user spent at least 50% more this month than last month, (you'll fetch from a DB all the user payments for the current month and the previous month).
Then you have to compose an e-mail message to the user that lists the categories for which spending was unusually high, with a subject like: "Unusual spending of $1076 detected!" and a body like:
Hello card user!
We have detected unusually high spending on your card in these categories:
* You spent $148 on groceries
* You spent $928 on travel
Love,
The Credit Card Company