`f. (2 5/11 - 3/22 : x) . 2 1/5 = 4 1/5`
`⇒ (27/11 - 3/22 : x) . 11/5 = 21/5`
`⇒ 27/11 - 3/22 : x = 21/5 : 11/5`
`⇒ 27/11 - 3/22 : x = 21/5 . 5/11`
`⇒ 27/11 - 3/22 : x = 21/11`
`⇒ 3/22 : x = 27/11 - 21/11`
`⇒ 3/22 : x = 6/11`
`⇒ x = 3/22 : 6/11`
`⇒ x = 3/22 . 11/6`
`⇒ x = 1/4`