`21/28 : 15/28 : 7/15` 2 - `3/4 : 3/5 ⨉ 2/5`

`21/28 : 15/28 : 7/15`

`= 21/28 xx 28/15 xx 15/7`

`= ( 21 xx 28 xx 15 )/( 28 xx 15 xx 7 )`

`= ( 3 xx 7 xx 28 xx 15 )/( 28 xx 15 xx 7 )`

`= 3`

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`2 - 3/4 : 3/5 xx 2/5`

`= 2 - 3/4 xx 5/3 xx 2/5`

`= 2 - ( 3 xx 5 xx 2 )/( 4 xx 3 xx 5 )`

`= 2 - ( 3 xx 5 xx 2 )/( 2 xx 2 xx 3 xx 5 )`

`= 2 - 1/2`

`= 4/2 - 1/2`

`= 3/2`