Answer:
b. Speedyland will double its GDP 21 years quicker than Slowland. 
Step-by-step explanation:
According to the rule of 70, it tells about the number of years to double 
For Slowlands
= 70 ÷ 2 
= 35
For speedyland
= 70 ÷ 5
= 14
So if we take the difference than it comes 
= 35 - 14
= 21
Hence, the correct option is b and the same is to be considered 
And all other options are wrong