Please Help With This Math Problem.

Question

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

Answer 1

This would be the difference been the number sent for 16 days ( 4x40=160) and the number sent through the first 9 days(3x40=120)

160 - 120= 40