A worker deposits a promotion bonus of $x in an interest bearing account at rate r1 compounded monthly. Six years later - the rate decreases to r2 - still compounded monthly. Another four years later - the worker consults a financial planner and learns that because of the change to the lower rate the account is now worth only 80% of what it could have been had the higher rate remained in effect for the entire period. What is the incremental difference between rates r1 and r2?