11 → (17 + 5) ÷ 2 = 113$21 = 12 → (3 + 21) ÷ 2 = 122$4 = 3 → (2 + 4) ÷ 2 = 3Similarly,42$6 = ? → (42 + 6) ÷ 2 = 24=> ? = 24Hence, ‘24’ is the correct answer.