Who Spent What?

A shop's till stores each order with a customer number. The names are in a separate customer list. A few orders come from numbers that aren't in the list yet, and the money from those orders still counts.

Task: write spend_by_customer(orders, customers).

• orders is a dictionary of two lists, "customer_id" and "amount", with one entry per order. There is at least one order.
• customers is a dictionary of two lists, "customer_id" and "name". Each customer number appears once, and every name is different.
• Return a dictionary from name to that customer's total spend, as a float rounded to 2 decimal places.
• Every order from a number that isn't in customers goes under the name "Unknown". If there are no such orders, there is no "Unknown" key.
• Customers with no orders are not included.