Alright, lets get started :
The area of sector is = 1/2 θ r^2 where θ is in radians
So lets first change our given angle θ ie 40 degree into radian.
For changing degree to radian, we multiply 40 * Pi/180 = 0.698 radians
Now lets plug the value in formula
Area = 1/2 θ r^2
Area = 1/2 * 0.698 * 15^2
Area = 78.54 square inches : Answer
Hope that will help :)