Bonjour,
(1/4 - 1/5) x (1 - 7/15)
= (1 x 5 / 4 x 5 - 1 x 4 / 5 x 4) x (1 x 15 / 1 x 15 - 7/15)
= (5/20 - 4/20) x (15/15 - 7/15)
= 1/20 x 8/15
= 8/300
= 4 x 2 / 4 x 75
= 2/75
- 1/3 + 2/7
1/2 - 7/8
= 1 x 7 / 3 x 7 + 2 x 3 / 7 x 3
1 x 4 / 2 x 4 - 7/8
= 7/21 + 6/21
4/8 - 7/8
= 13/21 : - 3/8
= 13/21 x - 8/3
= - 104/63