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Word problems are challenging, but rewarding once you learn how they're done!  Let's take a look at another word problem.  This problem deals interest rate percentages.  
Problem Text: A sum of $4000 is to be invested in 2 portions, on at 4%, the other at 6%.  If the interest earned with the portion at 4% exceeds the interest earned by the portion at 6% by $40, how much is invested at each rate?
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