According to today's foreign exchange rate, 100 Hong Kong dollars can be converted into RMB: 87.59 yuan (selling price).

Therefore, RMB 100 can be converted into Hong Kong dollars today:100/87.59×100 =14.17 (yuan).

If you want to change banks, show your visa to Hong Kong.

There is no visa to go to the black market, but the price is much higher. 100 yuan for 10 yuan is not bad.