Computer system organization cache memory

Question

Assume that A, B , and C are memory addresses each  of which are in different block of memory. Further assume  A, B , C are generated in a uniformly random way.

What is the probability that the second  instance  of "B" will be a hit on a 4-line direct-mapped cache?

Answer 1

Reference block A B C B

direct mapping

For A will map any of the 4 line as well as B also map any of the 4 line because In worst Case B also Map the same line in which A are mapped

and for C we have only 3 line out of 4 line because we dont't want  to remove A , and  because of that next reference of A will be hit....

probability will be 1*1*3/4  = 3/4