What is the percentage increase in sales in December 2017 as compared to the sales in December 2016?

28.57%

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We need to find December sales for both 2016 and 2017, then calculate the percentage increase.

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For 2016 Q4, total sales $= 360$ units and October $= 100$ units. The three months (Oct, Nov, Dec) form an arithmetic progression.

Let the AP be: $100, 100 + d, 100 + 2d$

Sum of AP $= 360$

$100 + (100 + d) + (100 + 2d) = 360$

$300 + 3d = 360$

$d = 20$

Therefore, December 2016 $= 100 + 2(20) = 140$ units

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For 2017 Q4, total sales $= 500$ units, October $= 150$ units, November $= 170$ units

December 2017 $= 500 - 150 - 170 = 180$ units

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Percentage increase $= \dfrac{\text{New Value} - \text{Old Value}}{\text{Old Value}} \times 100$

Percentage increase $= \dfrac{180 - 140}{140} \times 100$

Percentage increase $= \dfrac{40}{140} \times 100$

Percentage increase $= \dfrac{2}{7} \times 100 = 28.57\%$