Bonjour,
A) 4/5 + 2/5 x 15/4 =
4/5 + 15/5 x 2/4 =
4/5 + 3 x 1/2 =
4/5 + 3/2 =
(4 x 2)/(5x 2) + (3 x 5)/(2 x 5) =
8/10 + 15/10 = 23/10
B) (-4/7 - 5/7) x 14/(-21) =
(-9/7) x (-14/21) =
(-9/21) x (-14/7) =
(-3/7) x (-2) =
6/7
C) 4/3 x (6/5 - 19/15) =
4/3 x (18/15 - 19/15) =
4/3 x (-1/15) =
(-4/45)