The answer is actually b (since there is no greater than equal to sign you use a hollow dot instead of a full dot)
3x+x= 180°(straight angle)
4x= 180°
x= 180/4
x= 45°
Let the unknown number be x.
Given,






x =

= 1.28
Hence, the number is 1.28
The area of D is given by:

The average value of f over D is given by: