Using the same dating method (U235 and Pb207), the amount of Pb207 found in layer F is 96.875% and the amount ofU235 found in layer G is 6.

Using the same dating method (U235 and Pb207), the amount of Pb207 found in layer F is

96.875% and the amount of U235 found in layer G is 6.25%. For at least how many million years had the layer F been exposed on the surface to receive weathering and erosion before layer G started to form?