Years Purchase, dual rate              
no income tax            
(quarterly in advance)            
1            
4[1 - (1+r)-1/4] +  4[1-(1+a)-1/4]      
  (1+a)n -1      
Amount (£)      
Interest Rate (%)          
Term (years) (n)      
Sinking Fund (%)          
       
Answer (£)      
1     (1+r) (1+a)    
4[1 - (1+r)-1/4] +  4[1-(1+a)-1/4]   (1+r)^-1/4 (1+a)^n    
  (1+a)n -1   1-(1+r)^-1/4 (1+a)^n-1    
      4[1 - (1+r)^-1/4         
             
Effective yield [r] (1+i/p)^p -1   (1+a)      
p = no of times int     (1+a)-1/4        
is received and   [1 - (1+a)-1/4 ] 4[1-(1+a)-1/4]      
compounded      4[1-(1+a)-1/4] (1+a)n-1  
during the year