Buck Converter Efficiency

The duty cycle is a ratio of output voltage against input voltage. When given an output voltage of 20v and an input voltage of let us say 100 volts then the duty cycle is 20/100.

The duty cycle does not tell all about the efficiency.
The losses in the parts are not taken into account.
The switching element (most times a mosfet) has a resistance.
Also the used inductor will introduce losses.

To calculate the efficiency, you need to compute the ratio of the output power with respect to the input power. The immutable rule of DC-DC conversion schemes of any type is that the output power will ALWAYS be less than the input power. Sometimes it will be much less.

Edit: Here is an example from a simulation on an open loop buck converter. The figure of 89% is typical for rudimentary circuits. We can do better, but things get more complicated quickly.