1/3*5/12+1/3*1/4=1/3*(5/12+1/4)=1/3*7/12=7/36
(21+2/5)*5/7=21*5/7+2/5*5/7=15+2/7=105/7
12*(2/3+5/6)=12*2/3+12*5/6=8+10=18