ExplanationYou don’t know how many trees there are, so define the job by plugging in an easy number, such as 21 trees.

Use rate pies to figure out that Helen’s rate is 7 trees/hr and Sergey’s rate is 3 trees/hr. That means their combined rate is 10 trees/hr. Put their combined rate into another

rate pie with the total trees to find it takes them \(\frac{21}{10}\) or 2.1 hours to meet.

To find out how many trees Sergey has cut down at this point, put his individual rate into another rate pie with 2.1 hours. 3 trees/hr × 2.1 hours = 6.3 trees at the point they meet. Put the number of trees Sergey has cut when they meet over the total trees: \(\frac{63}{21}=\frac{3}{10}\), so the correct answer is \(\frac{3}{10}\) or 0.3.

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Sandy

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