Here is a frequency table of the billing amount for a given week ($000) for a sample of medical practices in metropolitan Adelaide:

Billing amount ($000's)	Frequency
(0, 20]	22
(20, 40]	17
(40, 60]	9
(60, 80]	11
(80, 100]	22

How many medical practices in this sample, had a weekly billing amount of more than $40,000 but less than or equal to $80,000?