1.
1/3+3/4+2/5+5/7+7/8+9/20+10/21+11/24+19/35
=1/3+3/4+2/5+5/7+7/8+(4/20+5/20)+(3/21+7/21)+ (3/24+8/24)+(5/35+14/35)
=1/3+3/4+2/5+5/7+7/8+1/5+1/4+1/7+1/3+1/8+1/3+2/5+1/7
=1/3*3+(3/4+1/4)+(2/5+1/5+2/5)+(5/7+1/7+1/7)+(7/8+1/8)
=1+1+1+1+1
=5
2.
(51+2/3)÷(5/3)+(71+3/4)÷(7/4)+(91+4/5)÷(9/5)
=(50+5/3)*(3/5)+(70+7/4)*(4/7)+(90+9/5)*(5/9)
=50*3/5+（5/3)(3/5)+70*4/7+(7/4)*(4/7)+90*5/9+(9/5)*(5/9)
=30+1+40+1+50+1
=123