Answer:
D
Step-by-step explanation:
When multiplying numbers with the same base but different exponents, add the exponents together.
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.
