We can use the scale formula to solve this problem. The formula is:
scale = map distance / actual distance
In this case, the map distance is 20 inches, and the actual distance is 3,000 miles. We need to convert these units so that they are both in the same units. Let's convert miles to inches:
1 mile = 63,360 inches (approximately)
So, 3,000 miles is equal to:
3,000 miles * 63,360 inches/mile = 190,080,000 inches
Now we can plug in the values into the formula:
scale = 20 inches / 190,080,000 inches
scale = 1 / 9,504,000
Therefore, the scale of the map is 1:9,504,000. This means that 1 inch on the map represents 9,504,000 inches (or 150 miles) in real life.