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 ~