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Accounts and notes receivable, net (Tables)
12 Months Ended
Dec. 31, 2021
Receivables [Abstract]  
Schedule of Accounts Receivable
     As of December 31,  
     2020      2021  
     RMB      RMB      US$  
Accounts and notes receivable
     88,706        81,550        12,797  
Less: allowance for doubtful accounts
     (43,820      (37,690      (5,914
    
 
 
    
 
 
    
 
 
 
Total accounts and notes receivable, net
     44,886        43,860        6,883  
    
 
 
    
 
 
    
 
 
 
Schedule of Movement in Allowance for Doubtful Accounts
The following table presents the movement in the allowance for doubtful accounts:
 
     As of December 31,  
     2020      2021  
     RMB      RMB      US$  
Balance at beginning of year
     28,516        43,820        6,876  
Provisions
     18,732        (246      (39
Write-offs
     (3,428      (5,884      (923
    
 
 
    
 
 
    
 
 
 
Balance at end of year
     43,820        37,690        5,914