Question

How much has the amount of carbon dioxide in the atmosphere increased since 1750

Answer 1

In 1750, the carbon dioxide levels were at a range of 275 to 280 parts per million. Now, there's about 400 parts per million in the atmosphere. 
275 to 400= 45%
280 to 400≈43%
The amount of carbon dioxide has increased by 43 to 45%.