Query in financial reporting

In one of the sums in FINANCIAL REPORTING, i came accross a sum for govt grants accounting, where grant was treated as pro-rata income as per the % of depreciation, that was charged on the asset. The method for calculating depreciation was WDV and the formula for WDV method was used to calculate the rate i.e. 1-nth root of (residualvalue/historical cost) where n = life in yrs.
I saw this sum, where life was 10 yrs.
If its 2 or 4 or 8 yrs its manageable, but how to calculate 10th root?? without log or with the calculator??