When and for how long is q > 900.
Let's find out when q = 900.
So we know that after day 5 q is greater than 900. If you look at the graph in part c) (which I have attached below), you will see that q never goes back down. So q > 900 for days 6, 7, 8, 9, and 10. Thus the answer is 5 days.