bonjour
soit a les minutes de com et b les SMS
30 a + 50 b = 19.5
50 a + 100 b = 35
- 60 a - 100 b = - 39
50 a + 100 b = 35
- 10 a = - 4
a = 4/10 = 0.4
min de com = 40 c
30 * 0.4 + 50 b = 19.5
12 + 50 b = 19.5
50 b = 19.5 - 12
50 b = 7.5
b = 0.15
SMS = 15 centimes