the population is large and about four fifths of it are farmers

population 做主语时,谓语一般用单数,如：
the population 【is】 large
但如果population 和分数,百分数连用,做主语,表语是复数时,此时谓语一般用复数,如：
about four fifths of it（=the population） 【are】 farmers（复数名词作表语）