You can put this solution on YOUR website! Let = Kate's age now = her father's age now
Kate's father is 50 years old. Kate is 24 years old now.
The difference in their ages will never change
So, I can say
Let = the number of years ago that
Kate's father was 3 times as old as her
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By substitution:
11 years ago Kate's father was 3 times her age
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check:
11 years ago, father was
and, 11 years ago,
The difference in their ages was:
OK