### Find the number of sides of a regular polygon whose each exterior angle is $40 ^ \circ.$

$9$
1. We know that the sum of all exterior angles of a regular polygon is $360 ^ \circ.$
2. It is given that each exterior angle is $40 ^ \circ.$
3. Thus, the number of sides in the given polygon $= \dfrac { \text{ Sum of measure of all exterior angles} } { \text{ Each exterior angle} }= \dfrac { 360 }{ 40 } = 9.$