The age gap is always 51 years. When Grandfather is 4 times as old, the gap is 3 units.<br><br>[[model:questions/solutions/q4192.png]]<br><br>• Gap → GF − Sam = 51 years always<br>• When GF = 4 × Sam: 4u − 1u = 3u → 51<br>• 1u → 51 ÷ 3 = 17 (Sam's future age)<br>• Years → 17 − 8 = 9<br><br>Sam's grandfather will be 4 times as old as him in 9 years.<br>Check: In 9 yrs, Sam 17, GF 68; 68 = 4 × 17; 68 − 17 = 51. ✓