Question 1203280
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Let x and 5x represent their ages when the father was 5 times as old as his son.  Then the difference in their ages then (and always) is 4x.<br>
Now their ages are 51 and 19, so the difference in their ages is 32. So<br>
4x = 32
x = 8<br>
Their ages when the father was 5 times as old as his son were x = 8 and 5x = 40.<br>
Since their ages now are 51 and 19, that was 11 years ago.<br>
ANSWER: 11<br>