If a freely falling rock were equipped with a speedometer, by how much would itd speed readings increase with each second if it were on a planet wreg=20m/s2?

" 20 m/s² " means that if gravity is the ONLY force on the object
(the object is in 'free fall'), then its speed increases by  20 m/s
every second.

That's the answer to your question.  Now, let me ask you
another one:

How does a speedometer tied to a falling rock work ?
How can it measure the rock's speed ?

Maybe one way would be to have a little tiny propeller on
the front of the speedometer, and it could measure how fast
the propeller is spinning as the rock falls through the air ?
Great idea.  But we already said the rock is in free-fall,
so there's no air resistance, we can't have any air, and
there's nothing to spin the propeller.

How would you do it ?  How can you measure the rock's speed ?

The sample with the lowest specific heat capacity will have the greatest temperature increase. in this case, it would be platinum.