解：这堆煤原来有 3/2÷(1/4-1/5) = 3/2÷1/20 = 30（吨）
1/4-1/5=1/203/2除1/20=30吨
1/4-1/5=1/20 3/ 2除以1/20=30
3/2÷(1/4-1/5)=30
3/2÷（1/4-15）=3/2÷1/20=30
