Chinese Numbers 10 – 100: Division in Chinese.

80 : 20 = 4

八十除以二十等于四。

bāshí chúyǐ èrshí děngyú sì

More information on the Chinese numbers you can find in The Chinese Numerals for Businessmen by Polina Shinkina http://www.lulu.com/content/e-book/the-chinese-numerals-for-businessmen/19124210 or in each Chinese textbook by Polina Shinkina on http://www.lulu.com/spotlight/polina985