The percentage, if the new cost was 84% of the original is 40%
Let the cost of the item be x.
When the price increased by y%, the new price is:

When the price decreased by the same percentage (i.e. y%), the new price is:

The cost is said to be 84% of the original price.
So, we have:

Divide both sides by x

Apply the difference of two squares

Subtract 1 from both sides of the equation

Cancel out the common factors

Take the square root of both sides

Multiply both sides by 100

Hence, the percentage is 40%
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