Bonjour,
1) (15 + 13 + 9 + 11) ÷ 4 = 12
2) a) (15 + 13 + 9×2 + 11×2) ÷ (4+2) = 11,333..... ≅ 11,3
b) (15 + 13 + 9×2 + 11×2 + note) ÷ (4+2+1) = 12
⇒ (68 + note) ÷ 7 = 12
⇒ 68 + note = 7 × 12 = 84
⇒ note = 84 - 68 = 16
