In a psychological study of short term memory, irrelevant speech effects refer to the degree to which the memorization process is impaired by irrelevant background speech For example, trying to memorize a list of numbers while listening to a speech in an unfamiliar language). An analysis of irrelevant speech effects was carried out and published in Acoustical Science & Technology (Vol. 35, 2014). Subjects performed the memorization task under two conditions: (1) with irrelevant background speech and (2) in silence. The difference in the error rates for the two conditions-called the relative difference in error rate (RDER)-was computed for each subject. A MINITAB histogram summarizing the RDER values for 71 subjects is displayed here.

a. Convert the frequency histogram into a relative frequency histogram.
b. What proportion of the subjects had RDER values between 75 and 105?
c. What proportion of the subjects had RDER values below 15?