To have at least 20 pencils left in the box, even after giving away 15% of the pencils, you would need to calculate the original number of pencils using the formula:
x - 15% = 20
Rearranging the equation:
x = 20 / (1 - 0.15)
Simplifying:
x = 20 / 0.85
x ≈ 23.53
Therefore, you would have needed originally around 23 or 24 pencils in your box.
b) What if you wanted to have at least 20 pencils left in your box? How many pencils would you have needed originally?
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