[9/10-（1/4+7/20）]/4/5
=[9/10-（1/4+7/20）]*5/4
=[9/10-7/20-1/4]*5/4
=[9/20-1/4]*5/4
=5/4*9/20-5/4*1/4
=9/16-5/16
=4/16
=1/4