Use of 成 / 乘 number quantifiers in Cantonese

Question

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:

1. When to use 成 VS 乘 ?
2. 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?

Answer 1

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 "(X)成" for "(X)/10"

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)

Answer 2

Tang Ho makes a clear explanation on the difference between 成/乘, but I would like to elaborate a bit more on the 2nd question.

Chinese people feel that odd numbers are “lucky numbers”, and tend to use them more when they assign numbers to certain cultural constructs. For example, Chinese emperors made their subjects build palaces with constant references to numbers like 5 and 9, and they were also referred to as 九五至尊. So this preference probably carried on to modern times.