Each boy appears in exactly two of the three pairs. So adding all three pair-totals counts every boy twice.<br><br>[[model:questions/solutions/q4179.png]]<br><br>• Add all pairs → 25 + 18 + 17 = 60<br>• This counts each boy twice → 2 × total = 60<br>• Total → 60 ÷ 2 = 30<br><br>The three boys have 30 cards in total.<br>Check: Paul 12, Simon 13, Gray 5 → 12+13=25, 13+5=18, 12+5=17; total 30. ✓