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Explanation:
15% of 15 = 0.15*15 = 2.25
The cost has increased by $2.25 which means the cost per kilogram is now 15+2.25 = 17.25 dollars
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As a slight shortcut, we can multiply the old cost (15) by the multiplier 1.15 to get the same result:
1.15*15 = 17.25
This method allows us to chain together many percentage increases, and it also allows us to include percentage decreases as well. Note how 1.15 is basically the result of 100% + 15% = 1.00+0.15 = 1.15