The age gap is 30 − 18 = 12 years and never changes. When Max was half, the gap is 1 unit.<br><br>[[model:questions/solutions/q4193.png]]<br><br>• Gap → 30 − 18 = 12<br>• When Max = half of brother: gap = 2u − 1u = 1u → 12<br>• 1u → 12 (Max's past age)<br>• Years ago → 18 − 12 = 6<br><br>Max was half his brother's age 6 years ago.<br>Check: 6 yrs ago, Max 12, Brother 24; 24 = 2 × 12; 24 − 12 = 12. ✓