Hi, as you are still confused I will try again.
You have 10 frames per second.
This means 10 frames in every second.
To work out the time for each frame, you need to split up the second into equal amounts.
Because you have 10 frames per second, you need to split the second up into 10 equal amounts.
this is done by division:
, this is 100 milliseconds using the conversion that you say you understand.