[originaltext] Not all math skills are learned in the classroom. Some of the

Not all math skills are learned in the classroom. Some of them come naturally. Consider the rapid calculations you make when you estimate the number of empty seats on the school bus or the number of cookies in a cookie jar. These rough estimates can often be done without counting. That’s because humans are born with the ability to approximate, or closely guess, the number of items in a group. Researchers refer to this quality as a person’s "number sense." Scientists have discovered that this inherent sense of numbers may influence learning and achievement in the classroom.(18)Studies with teenagers show that students who excel at estimating quantities also did well on standard math achievement tests. These results suggest a "strong and significant relationship" between a person’s inherent number sense and his or her ability to learn mathematics in school, says psychologist Justin Halberda of Johns Hopkins University. Researchers already knew that humans have a natural grasp of numbers. The ability to make rough estimate can be found in infants as young as four months old, and even in some animals.(19)This inherent number sense reaches back millions of years, researchers say, and has been used by humans and animals to help guide everyday behaviors such as hunting for food.
18. What does the passage say about students good at estimating quantities ?
19. Why do people need the ability to make rough estimates?
20. What is the passage mainly about?