【12分之5+8分之3-4分之1】×24
=5/12*24+3/8*24-1/4*24
=10+9-6
=13
13分之8÷7+7分之1×13分之6
=8/(13*7)+6/(13*7)
=14/(13*7)
=2/13
3×【15分之2+12分之1】-5分之2
=3*2/15+3*1/12-2/5
=2/5+1/4-2/5
=1/4