`S = 1/(2.5) + 1/(5.8) + 1/(8.11) + ... + 1/(17.20)`

`3S = 3/(2.5) + 3/(5.8) + 3/(8.11) + ... + 3/(17.20)`

`3S = (1/2 - 1/5) + (1/5 - 1/8) + (1/8 - 1/11) + ... + (1/17 - 1/20)`

`3S = 1/2 - 1/20`

`3S = 10/20 - 1/20`

`3S = 9/20`

`S = 9/20 : 3`

`S = 3/20`

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