20 marks what rate of return

I expects to have credit sales of 5,000,000 this yr. The bank offering me a lock-box system for 1,000 per month. i estimates that the new lock-box system will reduce float by 5 days.
Q1. what the rate of return must i earn on its marketable securities to make it worthwhile for the company to institute this lock-box system?
Q2. use a 365 day per yr.