I think this is right but it should be around this number 125 seconds
Suppose A Sequential File Contains 50,000 Records, And 5 Milliseconds Are Required To Interrogate An Entry. How Long Should We Expect To Wait When Retrieving A Record From The Middle Of The File?
To get to the middle of the file, you need to read through 25,000 records. At 5 ms each, your wait time will be
(25000 records)*(.005 seconds/record) = 125 seconds.
(25000 records)*(.005 seconds/record) = 125 seconds.