.
Decagon has 10 sides.
The sum of the interior angles of any convex polygon is  (n-2)*180  degrees, where n is the number of sides.
THEREFORE, for the regular decagon, the sum of its interior angles is  
    
 degrees = 8*180 degrees = 1440 degrees.
Hence, each individual interior angle is  1400/10 = 144  degrees.    ANSWER
Solved.