`(7/5 + x) - 2/5 = 4`
`⇔ 7/5 + x = 4 + 2/5`
`⇔ 7/5 + x = 22/5`
`⇔ x = {22}/5 - 7/5`
`⇔ x = 15/5`
`⇔ x = 3`
Vậy `x=3`
`1 3/4 + x - 7/8 = 5`
`⇔ 7/4 + x = 5 + 7/8`
`⇔ 7/4 + x = 47/8`
`⇔ x = 47/8 - 14/8`
`⇔ x = 33/8`
Vậy `x=33/8`
`(x - 2/3) : 2/5 = 1/4`
`⇔ x - 2/3 = 1/10`
`⇔ x = 23/30`
Vậy `x=23/30`
`1/30 + 1/42 + 1/56 + 1/72 + 1/81 + 1/90 + 1/110`
`= 1/30 + 1/42 + 1/56 + 1/72 + 1/90 + 1/110 + 1/81`
`= 1/{5.6} + 1/{6.7} + 1/{7.8} + 1/{8.9} + 1/{9.10} + 1/{10.11} + 1/81`
`= 1/5 - 1/6 + 1/6 - 1/7 + 1/7 - 1/8 + 1/8 - 1/9 + 1/9 - 1/10 + 1/10 - 1/11 + 1/81`
`= 1/5 - 1/11 + 1/81`
`= 541/4455`
