Is it possible to calculate the depreciation expense rate without the given lifespan of an item?

For example: Parent company bought a building from Subsidiary for 8,000,000. Original cost of the building was 10,000,000 with accumulated depreciation of 8,500,000. The building has a REMAINING life of 20 years, straight line depreciation is used.

I'm trying to figure out the original depreciation rate that the subsidiary company would use if they still owned the building.