Bonsoir ,
7/8 x 8/9 x 9/10 = 7/10 x 8/8 x 9/9 = 7/10 x 1 x1 = 7/10
1 min = 60 s
Donc 7/8 x 8/9 x 9/10 x 1 min = 7/10 x 60 s = 7 x 60 s / 10 = 7 x 6 s = 42 s