Explanation

(b)
Suppose that $\angle\ A = x,\ then\ \angle B = x + 10{^\circ},$
$\angle C = x + 20{^\circ}$ and $\angle D = x + 30{^\circ}$
So, we know that $\angle A + \angle B + \angle C + \angle D = 360{^\circ}$
On putting these values, we get
$(x) + (x + 10{^\circ}) + (x + 20{^\circ}) + (x + 30{^\circ}) = 360{^\circ}$
$\Rightarrow \ \ x = 75{^\circ}$
Hence, the angles of the quadrilateral are 75${^\circ},\ 85{^\circ},\ 95{^\circ},105{^\circ}$.