First, we have to find the rate of change for this problem. To do this, we subtract any two numbers of days and the coordinating two total questions. Let's just use the first two rows. 8-4=4, and 40-20=20. For every 4 days, he answers 20 questions. We can further simplify this by dividing both sides by 4. 4/4=1, and 20/4=5. For every 1 day, he answers 5 questions. Now, to find 16 days, we just multiply both sides by 16. 16*1=16 and 16*5=80. For every 16 days, he answers 80 questions. 80 is our answer.
