Which number illustrates the fact that in English, numbers do not all follow a regular naming pattern, thus making learning to count more difficult in English than in some other number systems?

At least two individuals have intentionally gone without sleep for over 200 hours. What did they experience as a result of their lengthy periods of sleep deprivation?

a) They had no serious long-term consequences. b) They suffered schizophrenic-like episodes for a year. c) They slept for over 48 hours immediately afterward. d) They experienced severe headaches and convulsions.

Under the normal curve the 50th percentile always falls at the

a. mean b. median c. mode d. all of these