此题是一道逆推题:
40÷(1-1/2)÷(1-1/4)÷(1-1/4)÷(1-1/5)÷(1-5/9)
=40÷1/2÷3/4÷3/4÷4/5÷4/9
=40×2×4/3×4/3×5/4×9/4
=400千克
答:这吨大米400千克.