4. Columbus had a hard time convincing anyone to undertake the expense of financing a voyage to an unknown place. Based on the information available from other sailors and partly on his own ideas, Columbus underestimated the circumference of the Earth by twenty-one percent. The circumference of the Earth is 24,901 miles at the Equator. What was Columbus' estimate of the Earth's circumference? Round your answer to the nearest mile.

19,671.79

Yes. That's the same answer I posted yesterday.

To find Columbus' estimated circumference of the Earth, we know that he underestimated the actual circumference by twenty-one percent.

First, we need to find twenty-one percent of 24,901 miles (the actual circumference at the Equator):
21% of 24,901 = (21/100) x 24,901 = 5,229.21 miles

Next, we subtract this value from the actual circumference to find Columbus' estimated circumference:
24,901 - 5,229.21 = 19,671.79 miles

Therefore, Columbus' estimate of the Earth's circumference was approximately 19,671.79 miles, rounded to the nearest mile as 19,672 miles.