=【5/6-（81/40-2/3）】÷7/8
=（5/6-81/40+2/3）÷7/8
=（5/6+2/3-81/40）÷7/8
=（3/2-81/40）÷7/8
=（60/40-81/40）÷7/8
= -21/40÷7/8
= -21/40×8/7
= -3/5