let the angles be 4x,x .interior and exterior angles make a linear pair so 4x+x=180 = 5x=180, x=36
which means the exterior angle is x=36 the sum of all exterior angles are 360 in a polygon and since its regular pultgon , all sides are equal so y can be the no of sides so,
36y=360
and y=10 which means the no of sides is 10