bonsoir
a = cahier et b = stylo
3 a + b = 900
5 a + 3 b = 1 700
b = 900 - 3 a
5 a + 3 ( 900 - 3 a) = 1 700
5 a + 2700 - 9 a = 1700
- 4 a = 1 700 - 2 700
- 4 a = - 1000
a = 250
3 * 250 + b = 900
750 + b = 900
b = 900 - 750
b = 150
cahier = 250 F et stylo = 150 F