Question 100568
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A boy is 4 years old and his sister is three times as old as he is. When the
boy is 12 years old, how old will his sister be?

>>...A boy is 4 years old and his sister is three times as old as he is...<< 

That tells us that the sister is three times 4, or 12.  She is therefore 8 
years older than her 4-year-old brother.

>>...When the boy is 12 years old, how old will his sister be?...<<

When the boy is 12, his sister will be 8 years older than him, so she'll be 20.

You don't need any algebra for that.  You just think it out. 

Edwin</pre>