Total mass = number of boxes × average mass.<br><br>
• Total mass of 12 boxes = 12 × 18.5 = 222 kg<br>
• Required total for 13 boxes = 13 × 20 = 260 kg<br>
• Mass of 13th box = 260 − 222 = 38 kg<br><br>
Final Answer: 38 kg