Jīnnián de xiāoshòu'é xiàjiàng le bǎi fēn zhī sìshí.
Sales dropped by forty percent this year.
Tips
grammar
Chinese percentages flip English word order: 百分之 ("of one hundred") comes BEFORE the number. So 40% = 百分之四十 ("of one hundred, forty"), not 四十百分之. Same pattern: 百分之五十 (50%), 百分之百 (100%).