1-1/2×{1-1/3×[1-1/4×(1-1/5)]} =1-1/2×{1-1/3×[1-1/4×4/5]} =1-1/2×{1-1/3×[1-1/5]} =1-1/2×{1-1/3×4/5} =1-1/2×{1-4/15} =1-1/2×11/15 =1-11/30 =19/30
