Bonjour,
D = 1/3 - 1/3 x 4/7
D = 1/3 - (1 x 4)/(3 x 7)
D = 1/3 - 4/21
D = (1 x 7)/(3 x 7) - 4/21
D = 7/21 - 4/21
D = 3/21
D = 1/7
