(3/20-2/15+1/3)÷2又4/5×4又1/5=(3/20-2/15+1/3)÷14/5×21/5=(3/20-2/15+1/3)×（5/14×21/5）=(3/20-2/15+1/3)×3/2=3/20×3/2-2/15×3/2+1/3×3/2=1/10-1/5+1/2=2/5
