`3/2 + 7/6 + 13/12 + 21/20 + ... + 91/90`
`= 1 + 1/2 + 1 + 1/6 + 1 + 1/12 + 1 + 1/20 ... +1/90`
`= 1+...+1 + ( 1/2+1/2.3+1/3.4 + 1/4.5 +...+1/9.10 )`
`= 1.9 + ( 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 +...+1/9 - 1/10 )`
`= 9 + ( 1 - 1/10 )`
`= 9 + ( 10/10 - 1/10 )`
`= 9 + 9/10`
`= 90/10 + 9/10`
`= 99/10`