(1又1/5+3又3/4+6.8+1/4)÷12/19
=(1又1/5+3又3/4+6又4/5+1/4)÷12/19
=[(1又1/5+6又4/5)+(3又3/4+1/4)]÷12/19
=(8+4)÷12/19
=12×19/12
=19