A mineral resource is extracted and sold, yielding £20m annual gross revenue to the owners. Purchases of goods and services used for extraction are £4m, labour costs are £2m and capital equipment is valued at £30m. The average rate of return on capital in the mineral extraction sector is 4.5%. At current extraction rates, reserves will be economically exhausted in 5 years. Assume a constant rate of extraction, a fixed extraction technology, and constant relative prices. Calculate a depletion rate for this mineral resource and hence the contribution of this extraction activity to gross and net national product, stating any necessary additional assumptions.