In how many ways can a garland of $20$ similar flowers be made?
A garland of $20$ similar flowers can be made in $\left(20 - 1\right)! = 19!$ ways.
This circular arrangement can either be clockwise or anticlockwise.
$\therefore \;$ Number of ways a garland of $20$ similar flowers can be made $= \dfrac{19!}{2}$ ways