`a) x - 1/3 = 1/7`
`x = 1/7 + 1/3`
`x= 10/21`
Vậy `x = 10/21`
``
`b) x - 1/24 = 5/8 - 1/31`
`x - 1/24 = 147/248`
`x = 147/248 + 1/24`
`x= 59/93`
Vậy `x = 59/93`
``
`c) x - 1/5 = 3/35 xx 7/6`
`x - 1/5 = 1/10`
`x = 1/10 + 1/5`
`x= 3/10`
``
`d) x xx ( 1/( 1 xx 2) + 1/( 2 xx 3) + 1/( 3 xx 4) + ... + 1/( 99 xx 100)) = 99/100`
`x xx ( 1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + .... + 1/99 - 1/100) = 99/100`
`x xx ( 1 - 1/100) = 99/100`
`x xx 99/100 = 99/100`
`x = 99/100 : 99/100`
`x = 1`
Vậy `x = 1`
``
`e) 720 : [ 41 - ( 2x - 5)] = 20`
`[ 41 - (2x - 5)] = 720 : 20`
`41 - (2x - 5) = 36`
`(2x - 5) = 41 - 36`
`2x - 5 = 5`
`2x = 5 + 5`
`2x = 10`
`x = 10 : 2`
`x = 5`
Vậy `x = 5`
