Réponses

2013-12-08T19:41:40+01:00
Appelons n le nombre cherché
(1+7+11+19+30+n) / 6 = 3n
(1+7+11+19+30+n)= 3n * 6
68 + n = 18 n
68 = 17 n
n = 68/17 = 4