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Question about Littles Law

I know the Little Law states (rephrases):

the average number of things in the system is the product of the average speed at which things leave the system, and the average time that each of them spends in the system, or:

n=x*(r+z);
x-throughput
r-response time
z-think time
r+z - average response time

now I have a question about the problem with pearl programming:

Suppose a system makes 100 disk accesses to process a transaction (although some systems require less, some systems will require several hundred disks for each transaction). How many transactions per hour per disk can the system process? Assumption: disk access takes 20 milliseconds.

Here is the solution to this problem

- , 20 ( ) 2 1800

, ,

+3
1

, , , - , , . , 0,5 , , 2 , 1/0,5 = 2.

, - . , 100 , 20 . 2 . 1/2 = 0,5 .

, :

R TT.

R = 1/TT

TT :

TT = disk access time * number of disk accesses per transaction =
20 milliseconds * 100 = 2000 milliseconds = 2 seconds

R = 1/2 transactions per second 
= 3600/2 transactions per hour
= 1800 transactions per hour
+3

Source: https://habr.com/ru/post/1748851/

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