"A is 4/5 slower than C" is actually "A is 4/5 slower than C". -to put it bluntly, it is "a fraction of who you are compared with."

Then the question is: C-4/5C=A, that is, a = (1/5) c.

"A is twice as slow as C" can't be said. Can only say "A is twice as fast as C" or "A is half as slow as C". That is to say:

A is slower, less and lower than C by several scores (less than1);

A is faster than C, many times (more than 1).

Of course, the above are all fractions or multiples of C. ...

According to this idea, I will understand when I see you later ~