Sous la forme la plus simple: 23 sur 36 -1 sur 9 +7 sur 18 7 sur 12 - 13 sur 24 + 5 sur 3 7 sur 5 - 17 sur 5 x 12 sur 51 7 sur 3 x 5 sur 4 -11 sur 15 x 25 sur 2

23/36  - 1/9 + 7/18
= 23/36 - 1 x 4 / 9 x 4 + 7 x 2 / 18 x 2
= 23/36 - 4/36 + 14/36
= 33/36
= 3 x 11 / 3 x 12
= 11/12

7/12 - 13/24 + 5/3
= 7 x 2 / 12 x 2 - 13/24 + 5 x 8 / 3 x 8
= 14/24 - 13/24 + 40/24
= 41/24

7/5 - 17/5 x 12/51
= 7/5 - 204/255
= 7 x 51 / 5 x 51- 204/255
= 357/255 - 204/255
= 153/255
= 51 x 3 / 51 x 5
= 3/5

7/3 x 5/4 - 11/15 x 25/22
= 35/12 - 275/330
= 35 x 330 / 12 x 330 - 275 x 12 / 330 x 12
= 11550/3960 - 3300/3960
= 8250/ 3960
= 330 x 25 / 330 x 12
= 25/12

(2 - 5/7) x 28/45
= (2 x 7 / 1 x 7 - 5/7) x 28/45
= (14/7 - 5/7) x 28/45
= 9/7 x 28/45
= 252/315
= 63 x 4 / 63 x 5
= 4/5

7/4 - 15/7 x 21/40
= 7/4 - 315/280
= 7 x 70 / 4 x 70 - 315/280
= 490/280 - 315/280
= 175/280
= 35 x 5 / 35 x 8
= 5/8

/ = sur

23/36 - 1/9 + 7/18
=46/72 - 8/72 + 28/72 |( 36x2 | 9x8 |18x4)
=66/72 
=11/12 ( on divise par 6)

7/12 - 13/24 + 5/3
=14/24 - 13/24 + 40/24 (12x2 | 3x8)
=41/24

7/5 - 17/5 x 12/51
=7/5 - 204/255 ( on divise par 51 )
=7/5 - 4/5
=3/5

7/3 x 5/4 - 11/15 x 25/22
=35/12 - 275/330 (330 x 25 | 330 x 12)
=25/12

(2 - 5/7) x 28/45
= (2 x 7 / 1 x 7 - 5/7) x 28/45
= (14/7 - 5/7) x 28/45
= 9/7 x 28/45
= 252/315 (63 x 4 | 63 x 5 )
= 4/5

7/4 - 15/7 x 21/40
= 7/4 - 315/280
= 7 x 70 / 4 x 70 - 315/280
= 490/280 - 315/280
= 175/280  (35 x 5 | 35 x 8)
= 5/8