watch this video for concept https://www.youtube.com/watch?v=rT4eI3p3tVk
Here, S is outer relation and R is inner relation. For a minimum number of block accesses, outer relation should be smaller.
Number of records in each block = (Size of Block)/(Size of Record) = 100B/5B = 20
NR = Total Number of records in R = 2000
BR = Number of blocks in R = 2000/20 = 100
NS = 500
BS = 500/20 =25
Number of total block accesses = BS + BS*BR (for Block nested loop join) = 25 + 25*100 = 2525