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?