120÷24-1.5×1.2
=5-1.8
=3.2
8/9×{3/4-(9/10-3/8)}
=8/9X{（30/40-（36/40-15/40）}
=8/9X9/40
=1/5
315+540÷12×13
=315+45X13
=315+585
=900