Secondary composite-key pages access
Since the index (EID,Salary) has a height of h, the PK on EID must have height &lt;=h.  If this is a clustered index, then for &lt;= h LIOs (page reads) the PK can satisfy the query.  

Then assuming the PK is not clustered, using the PK costs h or h + 1 LIOs.


  condition will be checked and we will either get this employee or not. So how can I calculate the costs of each in order to compare between them? 


The probability that the salary >60000 is about 0.5, so you can calculate an expected cost.  Using the index on (EID,Salary) has an expected cost of
0.5 * h + 0.5 * (h+1) = h + 0.5 LIOs.

Using the (Salary,EID) index costs h LIOs to seek to the first row with Salary >60000, and then you can scan across the leaf pages to find EID=5, which will require reading (0 -.05] of the total leaf pages.  If Employee has 2 rows per page the (Salary, EID) index would have, perhaps, 8 rows per page.  So you scan, on average, .25 of the rows or (20,000/8)/4=625.  So that would cost, on average 625+h LIOs.