Bonjour,
a) 1/5-8/15×5/4
= 1/5 - 40/60
= 1 x 12 / 5 x 12 - 40/60
= 12/60 - 40/60
= - 28/60
= 4 x - 7 / 4 x 15
= - 7/15
c) -4/7:5/14+5/6
= - 4/7 x 14/5 + 5/6
= - 56/35 + 5/6
= - 56 x 6 / 35 x 6 + 5 x 35 / 6 x 35
= - 336/210 + 175/210
= - 161/210
= 7 x - 23 / 7 x 30
= - 23/30