There wasn't much to go off of but based of the information you've gave me the answer would be 7.68=6.28
To solve the problem, we need to get the formula for area of a circle -
Area = pi * r^2; r= radius
= 3.14 * (13cm)^2
= 3.14 * 169cm^2 or 3.14 * 13^2cm^2
= 530.66cm^2
Therefore, the area of the circular plate is 530.66cm^2
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.
