`a,3/7+(-3/2)+(-3/5)` $\\$`=30/70-105/70-42/70`$\\$ `=-117/70`
`b,4/5-(-2/7)-7/10`$\\$ `=4/5+2/7-7/10` $\\$`=56/70+20/70-49/70` $\\$`=27/70`
`c,2/3-[-7/4-(1/2+3/8)]` $\\$`=2/3+7/4+(4/8+3/8)` $\\$`=8/12+21/12+7/8` $\\$`=29/12+7/8` $\\$`=58/24+21/24` $\\$`=79/24`