Amount of fuel used by one car to travel 5000 miles=5000/20=250 gallons.
Each gallon produces 9.5kg of CO₂, so 250 gallons produce 9.5×250=2375 kg of CO₂.
For 40,000,000 cars this comes to 9.5×10¹⁰ or 95 billion kg of CO₂. This is the same as 95 million metric tonnes.
(2375=2.375×10³ and 40,000,000=4×10⁷, so the product is 4×2.375×10¹⁰=9.5×10¹⁰.)