Unless the question specifically asks for it, it's not necessary to use here.

Let the initial cost be , or . We don't actually need to know what the initial cost is, we're only interested in its percentage change.

Then, the cost after months is given, as a percentage of the initial cost, by .

will give the cost after 12 months as a percentage of the original. Subtract from it to get the increase in percentage cost.

The time required for the cost to double can be found by setting and solving for .

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