The whole class likes Chinese or math, accounting for1-1/20 =19/20 (minus those who don't like it, they like at least one subject).

Then there are 4/5+3/4=3 1/20 who like math and Chinese, and there are twice as many people who like math and Chinese (some like Chinese and some like math), so it is 1 more, so the proportion of people who like math and Chinese is 3 1/20-.

Or 4/5+3/4+ 1/20=32/20=8/5, because there are twice as many people who like math and Chinese (some like Chinese and some like Chinese), so 8/5- 1=3/5 is the proportion of people who like both.