[9/10-(1/4+7/20)]÷4/5
=(9/10-1/4-7/20)÷4/5
=(18/20-7/20-5/20)×5/4
=3/10×5/4
=3/86.8×（3.2-2x）=03.2-2x=02x=3.2x=3.2/2x=1.6