The density value for the 20 to < 40 minute period is the ratio of the frequency to the total frequency of the distribution, which is 0.146
The Density for the 20 to < 40 minute period is given by the relative frequency value of the given length of time :
Relative frequency = Frequency / Cummulative frequency
Frequency of the 20 to < 40 period = 31
The Density value for the period is :
Therefore, the density value for the 20 to less than 40 period is 0.146.
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