The proper equation would be 20h, as if Andrea reads 20 pages per hour, you simply multiply by the number of hours she's been reading to find the number of pages she's read.
For example, if Andrea was to read for 3 hours, she would have read 60 pages as 20*3 = 60
The greenhouse effect is earths atmosphere being polluted by gasses that prevent heat from escaping as easily causing earth's temperature to rise and if that didnt exist earth would probably be alot cooler.
