Problem $6\mathrm{.}9$: Brianna has done a pretty good job planning for her future, but she knows retirement is not that far off and she needs to update her financial plans. Brianna plans to retire in $10$ years and currently has $$350,000$ in a bond fund and $$500,000$ in a stock fund. In addition, she plans to invest $$10,000$ per year into her stock fund at the end of each of the next $4$ years, and then up the amount to $$12,000$ per year at the end of the final $6$ years until she retires.

a$.$ Assuming Brianna's bond fund returns $5\%$ compounded annually, and her stock fund will return $9\%$ compounded annually, how much will Brianna have at the end of $10$ years?

b$.$ If Brianna expects to live for $25$ years after she retires, and at retirement she deposits all of her bond and stock investments in a mixed retirement account paying $6\%,$ how much can she withdrawal each year after she retires $($making $25$ equal withdrawals beginning one year after she retires$)$ so that Brianna ends up with a zero balance at death? solve in excel