a + bc

= 3/4 + 2/3x4/5

= 3/4 + 8/15 = 45/60 + 32/60 = 77/60

(c-a) x b

(4/5 - 3/4) x 2/3

(16/20 - 15/20) x 2/3

1/20 x 2/3 = 2/60 = 1/30

axbxc

= 3/4 x 2/3 x 4/5

tu peux le faire :)