Answer:
At the growth rate of 3% per year
Number of years taken to double the GDP = 23.33 years
The the GDP will double ( 23.33 - 20 ) 3.33 years earlier at 3.5% growth rate
Explanation:
According to the rule of 70
Number of years taken to double the GDP = 70 ÷ [ Growth rate ]
Thus,
At the growth rate of 3% per year
Number of years taken to double the GDP = 70 ÷ 3
= 23.33 years
Further 
if the growth rate is 3.5% per year
Number of years taken to double the GDP = 70 ÷ 3.5
= 20 years
Hence,
The the GDP will double ( 23.33 - 20 ) 3.33 years earlier at 3.5% growth rate
 
        
             
        
        
        
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