Réponse :

Bonsoir

Explications étape par étape

39/14 x 21/78 = (39 x 3 x 7)/(2 x 7 x 39 x 2) = 3/(2 x 2) = 3/4

40/11 x 55/60 = (2 x 20 x 5 x 11)/(11 x 3 x 20) = (2 x 5)/3 = 10/3

12/14 x 21/30 = (3 x 4 x 3 x 7)/(2 x 7 x 3 x 10) = (3 x 4)/(2 x 10) = (3 x 4)/(4 x 5) = 3/5