Without ignoring the variable costs, calculate $($a$)$ the ARPU and $($b$)$ the total user base needed for Dropbox to break even over a year if its Fixed Costs are $$400$ Million per year, Conversion Rate is $5\%,$ Average Cost to support a Free User is $$10$ per year, Average Cost to Support a Paying User is $$100$ per year, and Average Revenue Per Paying User is $$690$ per year? $($Assume negligible churn$).$