This is the application of ratio and proportion. You have to formulate the proportion for this problem:
50 deliveries : 5 days = 400 deliveries : x days
The inner numbers when multiplies should equal the product of the outer numbers.
400 (5) = 50x
50x = 2,000
x = 40
Therefore, it would take 40 days for the trucking company to deliver 400 deliveries.