`I = \frac{1}{1.2.3} + \frac{1}{2.3.4} + ... + \frac{1}{18.19.20}`
`=> I = 1/4( \frac{4}{1.2.3} + \frac{4}{2.3.4} + ... + \frac{4}{18.19.20} )`
`=> I = 1/4( \frac{1}{1.2} - \frac{1}{2.3} + \frac{1}{2.3} - \frac{1}{3.4} + ... + \frac{1}{18.19} - \frac{1}{19.20})`
`=> I = 1/4( 1/2 - \frac{1}{19.20} )`
`=> I = 1/4( 1/2 - 1/380 )`
`=> I = 1/4 . 189/380`
`=> I = 189/1520 `