Use of 成 / 乘 number quantifiers in Cantonese I learned in Cantonese class that adding 成 to a Number can imply a degree of something. For example, 七成 means 70%.

I explored further and it seems that it can be used to describe the rarity of your steak. Such that:

一成熟：rare (10% cooked)
三乘熟：medium rare (30% cooked)
五乘熟：medium (50% cooked)
七乘熟：medium well (70% cooked)
全熟：Well done (100% cooked)

I had 2 questions here:


When to use 成 VS 乘 ?
Why is this particular set of numbers (10%, 30%, 50%) chosen? 
I have seen something similar when choosing amount of sugar in bubble tea (i.e. 10%, 30%, 50%, 70% and 100%). Why not choose 25%, 50%, 75% instead?
成: one tenth; 10 per cent
乘: multiply; multiplication

一成熟：rare (10% cooked) 三成熟：medium rare (30% cooked) 五成熟：medium (50% cooked) 七成熟：medium well (70% cooked) 全熟：Well done (100% cooked)
It is always &quot;(X)成&quot;  for &quot;(X)/10&quot;
一成 = 1/10
三成 = 3/10
五成 = 5/10
七成 = 7/10
十成 = 10/10
十二成 = 12/10 (overdrive)