a + b = 33 si a + c = 11
5 × a + 3 × b + 2 × c = 121
= 3 × a + 3 × b + 2 × a + 2 × c =
= 3 × ( a + b ) + 2 × ( a + c ) =
= 3 × 33 + 2 × 11 =
= 99 + 22 =
= 121
