( 2/3 - 1/2) : ( 1/3 - 1/5) =
1/10 : ( 2/3 - 3/5 ) =
5/3 - ( 2/5 · 7/2 - 1/3 )
5/3 - ( 2/5 · 7/2 ) - 1/3 =
1/5 x ( 1/2 + 1/3 ) =
5 / 4
- ( 1 / 15)
3 / 5
1 / 6
3 / 2
