18*(1/15+1/21)*105
=(1/15+1/21)*15*7*3*6
=(1/15+1/21)*15*21*6
=21*6+15*6
=(21+15)*6
=36*6=216
4/5-3/8+6/5+3/8
=(4/5+6/5)+(3/8-3/8)
=2