The mouse, for example, has seen buttons added to it by gaming companies, and buttons subtracted from it by Apple.

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The actual number of putts taken by a player is subtracted from this average value to determine strokes gained or lost.

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In the example, the spread is 0.15% (that is the bid subtracted from the ask, divided by the share price and multiplied by 100).

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We also discounted the portion of Ikea's sales that we estimate are earmarked for a charitable trust and subtracted a percentage of the sales that are held by another Kamprad company.

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We subtracted neither taxes on this income, which will presumably be sheltered by depreciation deductions, nor operating costs. (At any one time, Don Jr. has a half-dozen people helping him on the licensing operation.) But neither did we add for the potential from future deals not yet signed, the going concern value of the operation.

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After determining the initial sex ratios of a series of clutches, the researchers added or subtracted chicks to increase or decrease the father's workload (a standard trick in avian ecology, made possible by the fact that parents do not recognise their own young until close to the time at which they leave the nest).

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