（1.5+5/3）÷2.5-0.5=(3/2+5/3)÷5/2-1/2=(3/2+5/3)×2/5-1/2=3/2×2/5+5/3×2/5-1/2=3/5+2/3-1/2= 18/30+20/30-15/30=23/30
