Your aunt has $$640,000$ invested at $4\mathrm{.}9\%,$ and she now wants to retire. She wants to withdraw $$45,000$ at the beginning of each year, beginning immediately. She also wants to have $$50,000$ left to give you when she ceases to withdraw funds from the account. For how many years can she make the $$45,000$ withdrawals and still have $$50,000$ left in the end?