1/2-(6/7-5/8)-1/7
=1/2-6/7+5/8-1/7
=(1/2+5/8)-(6/7+1/7)
=9/8-1
=1/8
2/21+(2/3-2/7)+6/7
=2/21+2/3-2/7+6/7
=(2/21+2/3)+(6/7-2/7)
=17/21+4/7
=29/21
1/2-(6/7-5/8)-1/7 =1/2-6/7+5/8-1/7 =(1/2+5/8)-(6/7+1/7) =9/8-1 =1/8 2/21+(2/3-2/7)+6/7 =2/21+2/3-2/7+6/7 =(2/21+2/3)+(6/7-2/7) =17/21+4/7 =29/21