Is there any difference between 成 and 百分之 when it comes to telling about percentage? I've just encountered 成 for the first time and started wondering.
 [n] one tenth; 10 per cent
so it's usually used with round numbers where you could use 十分之~
一成股權 = 10% equity
牛排烤到七成熟 = Steak grilled to 70% done
十成功力 = 100% strength
but in the following format can be used for exact percentages
一成三的股權 = 13% of equity
九成九是他 = 99% certain it is him
The only real place where you'll see an important difference is when you're talking about 20%. Because 成 is technically talking about tenths as a kind of thing, you figure 百分之二十 but you count 两成.
一成 means 10% 百分之 means %
Note that 成 has many meanings, and it could be confusing:
● 成chéng ㄔㄥˊ ◎ 做好，做完：成功。完成。成就。成事。成交。成立。成婚。成仁（儒家主张的成就仁德）。成人之美。玉成其事。 ◎ 事物发展到一定的形态或状况：成形。成性。成人。自学成才。蔚然成风。 ◎ 变为：长成。变成。 ◎ 可以，能行：成，就这么办。 ◎ 称赞人能力强：他办事麻利，真成。 ◎ 够，达到一定数量：成年累（lěi ）月。 ◎ 已定的，定形的：成规。成俗。成见。成例。成竹在胸。 ◎ 十分之一：增产三成。 ◎ 平定，讲和：“会于稷，以成宋乱”。 ◎ 姓。
成 means "perdec" (per ten, a tenth).
百分之 means "percent" (per hundred, a hundredth).
千分之 means "per mille" (per thousand, a thousandth).
三成 = three perdec = 3/10 (i.e. 30%)
七成 = seven perdec = 7/10 (i.e. 70%)
百分之三 = three percent = 3%
百分之七 = seven percent = 7%
千分之三 = three per mille = 3‰
千分之七 = seven per mille = 7‰