Answer:
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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.
21/25
Change to decimal:-
Divide numerator by denominator.
21/25
21 ÷ 25 = 0.84
21/25 = 0.84
Change to a percentage:-
0.84 × 100 = 84
21/25 = 0.84 = 84%