`D=1 1/21 :(15,75 - 15 1/4)+2 1/12 : (7 3/4 - 7,25)`
`D=22/21: (63/4 - 61/4 ) + 25/12 : (31/4 - 29/4)`
`D= 22/21 : 2/4 + 25/12 : 2/4`
`D= 22/21 : 1/2 + 25/12 : 1/2`
`D= 22/21 × 2 + 25/12 × 2 `
`D= ( 22/21 + 25/12 ) × 2`
`D= (88/84 + 175/84) × 2`
`D= 263/84 × 2`
`D= 526/84 = \frac{526:2}{84:2} = 263/42.`