Salut !
5/6 - 1/3 = 5/6 - (1×2)/(3×2) = 5/6 - 2/6 = 3/6 = (3÷3)/(6÷3) = 1/2
3/4 + 7/24 = (3×6)/(4×6) + 7/24 = 18/24 + 7/24 = 25/24
-8/45 - 2/5 = -8/45 - (2×9)/(5×9) = -8/45 - 18/45 = -26/45
9/14 - 5 = 9/14 - (5×14)/14 = 9/14 - 70/14 = -61/14
1/6 - 17/30 + 4/5 = (1×5)/6×5) - 17/30 + (4×6)/(5×6)
= 5/30 - 17/30 + 24/30
= 12/30
= (12÷6) / (30÷6)
= 2/5
21/25 - 3/5 = 21/25 - (3×5)/(5×5) = 21/25 - 15/25 = 6/25
