You can
put this solution on YOUR website!Good try, but incorrect. If they both walked a total of 15 miles and Susan walked 12 (according to your calculation), then Erin would have walked 9 miles (12-3=9)... but these answers would put us over the total miles walked.
So, here is how is done:
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Since Erinwalked 3 miles less than Susan and their sum is 15 miles...
Let Susan = x
and Erin = x-3
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So the equation now becomes:

Does it make sense?
Now solve for x to determine how many miles Susan walked.
Then deduct 3 to determine how many miles Erin walked.
Their sum must equal 15 miles.
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Done! :)
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PS. Erin walked 6 miles.