4 × (1 + 1/4) ÷ (1 - 1/5 - 1/5 × 1/4)
= 4 × 5/4 ÷ (4/5 - 1/20)
= 5 ÷ 3/4
= 20/3吨
答：这堆煤有3分之20吨.