Kylie is a psychologist who studies memory. She wants to see whether background music affects memorization. Kylie randomly sorts 40 subjects into two groups. The 20 subjects in one group are given five minutes to memorize a list of words while background music is playing. The 20 members of the other group perform the same task without any background noise. Afterward, Kylie records how many words from the list each subject could remember.
Words memorized with background music
929149272820111323251622261414211 31812
Mean: 18.2 words
Words memorized without background music
282428242726137151227629202226222 0179

How much lower is the average for the treatment group than the average for the control group?