Salut ! :)
On va faire tous les cas possibles :
n peut prendre les valeurs 1 ; 2 ; 3 ; 4 ; 5 ; 6 ; 7 ; 8 ; 9 ; 10
n = 1
a = 5+1 = 6
b = 2×1 = 2
n = 2
a = 5+2 = 7
b = 2×2 = 4
n = 3
a = 5+3 = 8
b = 2×3 = 6
n = 4
a = 5+4 = 9
b = 2×4 = 8
n = 5
a = 5+5 = 10
b = 2×5 = 10
n = 6
a = 5+6 = 11
b = 2×6 = 12
n = 7
a = 5+7 = 12
b = 2×7 = 14
n = 8
a = 5+8 = 13
b = 2×8 = 16
n = 9
a = 5+9 = 14
b = 2×9 = 18
n = 10
a = 5+10 = 15
b = 2×10 = 20
Donc sur les 10 cas possibles, un seul permet d'obtenir a = b, donc la probabilité est de 1/10
Voilà, j'espère que tu as compris. :)
