Question

Cell phone companies estimate that the total number of text messages (t) since the beginning of the month can be modeled by t=40×sqrt(x) where (x) is the number of days passed in the month. How many text messages were sent between the ninth and sixteenth day of the month?

Answer 1

The desired number is the difference between the number of messages sent by the 16th day and the number sent by the 9th day:

... n = t(16) - t(9)

... n = (40×√16) - (40×√9)

... n = 40×(4 - 3) = 40

40 text messages were sent between the ninth and sixteenth day of the month.