Answer:
20
Step-by-step explanation:
Let the number of sides be n
One exterior angle = 360/n
Exterior angle + interior angle = 180
Interior angle = 180 - Exterior angle
= 180 - 360/n
Sum of interior angle = n(180 - 360/n) = 3240
n(180 - 360/n) = 3240
180n - 360 = 3240
180n = 3240 - 360
= 3600
n = 3600/180
= 20
