Kelvin has a part-time job picking apples. Each day, he starts with an empty container, which can hold \(960\) apples, and puts the apples that he picks into the container. Kelvin is finished his work for the day if either \(3\) hours have passed or he has filled the container.
Kelvin picks \(400\) apples on the first day, and each day steadily improves the rate at which he picks apples by picking \(21\) more apples than the day before.
After how many days of working can he fill the container in less than \(3\) hours?
Theme: Algebra