I think what you meant to say is "If a triangle has 2 sides that are 4 cm and 20 cm, what are the possible lengths of the third side?"
Basically, when given 2 sides of a triangle, the third side must be GREATER than the DIFFERENCE of the other 2 sides and LESS than the SUM of the other 2 sides.
difference = 16 cm
sum = 24 cm
So, the third side must be greater than 16 cm and less than 24 cm
