Answer:
The rate at which the population decrease is 59.8 %  
Step-by-step explanation:
Given as :
The population of city = 30,31,000
Now The population of city decrease to 22,46,000
The time period in which population decrease is 50 years 
Let the percentage rate of decrease = R%
So, 
Final population = initial population × 
Or, 22,46,000 = 30,31,000 × 
Or,  =
 =   
 
Or,  =
 =  
 
 =
 =  
 
So , 0.99402 =  
 
Or,   = 1 - 0.99402
 = 1 - 0.99402
So, Rate =  × 100
 × 100
Or, Rate = 0.598 = 59.8 %
Hence The rate at which the population decrease is 59.8 %  Answer