Answer
The length of the base must be between $0$ and $20~cm$
Work Step by Step
If the legs are close together and the vertex angle is close to $0^{\circ}$, the length of the base will be close to zero.
If the legs are far apart and the vertex angle is close to $180^{\circ}$, the length of the base will be close to the sum of the two legs, that is $20~cm$.
The length of the base must be between $0$ and $20~cm$
