The obvious one is (310 beats)/(1 minute).
 
        
             
        
        
        
9 - 18
5 - 10
1 - 2
Work —
First line:
y = 2 (9)
y = 18
Second line:
10 = 2x
5 = x
Third line:
y = 2 (1)
y = 2
        
                    
             
        
        
        
After every month's withdrawals, 4/5 of the original amount at the start of the month will remain. The amount at the start of every month will change. Thus:
an = 4/5 (an-1) ; where a1 = 500.