At the time that an article was first featured on the home page of a news website, there were 40 comments on the article. An exponential model estimates that at the end of each hour after the article was first featured on the home page, the number of comments on the article had increased by 190% of the number of comments on the article at the end of the previous hour. Which of the following equations best represents this model, where C is the estimated number of comments on the article t hours after the article was first featured on the home page and t ≤ 4?

Explanation

Choice D is correct. It's given that an exponential model estimates that the number of comments on an article increases by a fixed percentage at the end of each hour. Therefore, the model can be represented by an exponential equation of the form C = Kat, where C is the estimated number of comments on the article t hours after the article was first featured on the home page and K and a are constants. It's also given that when the article was first featured on the home page of the news website, there were 40 comments on the article. This means that when t = 0, C=40. Substituting 0 for t and 40 for C in the equation C = Kat yields 40 = Kao, or 40=K. It's also given that the number of comments on the article at the end of an hour had increased by 190% from the number of comments on the article at the end of the previous hour. Multiplying the percent increase by the number of comments on the article at the end of the previous hour yields the number of estimated additional comments the article has on its home page: (40)(190/100), or 76 comments. Thus, the estimated number of comments for the following hour is the sum of the comments from the end of the previous hour and the number of additional comments, which is 40+76, or 116. This means that when t=1, C=116. Substituting 1 for t, 116 for C, and 40 for K in equation C = Kat yields 116 = 40a1, or 116 = 40a. Dividing both sides of this equation by 40 yields 2.9 = a. Substituting 40 for K and 2 9. for a in the equation C = Kat yields C = 40(2.9)t. Thus, the equation that best represents this model is C = 40(2.9)t.