Question 1152508
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The difference in their ages is 6 years.<br>
When Bill was three times as old as Sam, their ages were 3x and x; the difference is 2x.<br>
So when Bill was three times as old as Sam, we have<br>
{{{2x = 6}}}
{{{x = 3}}}<br>
So it was when Sam was x=3 years old and Bill was 3x=9 years old that Bill was 3 times as old as Sam.<br>
And since their ages are now 18 and 24, that was 15 years ago.<br>