`b, 2/5+6/9+3/4+3/5+1/3+1/4`
`= (2/5 + 3/5) + (3/4 + 1/4) + (6/9 + 1/3)`
`= 5/5 + 4/4 + 9/9`
`= 1+1+1`
`=3`
`c, 1/10+4/20+9/30+16/40+25/50+36/60+49/70+64/80+81/90`
`= (1/10 + 81/90) + (4/20 + 64/80) + (9/30 + 49/70) + (16/40+36/60) + 25/50`
`= (9/90 + 81/90) + (16/80 + 64/80) + (3/10 + 49/70) + (4/10+6/10) + 1/2`
`= 90/90 + 80/80 + 70/70 + 10/10 + 1/2`
`= 1+1+1+1+0,5`
`= 4,5`