Binary threshold finding?

if i have 2 matrices:

A=

    17    24     1     8    15     
    23     5     7    14    16
     4     6    13    20    22
    10    12    19    21     3
    11    18    25     2     9

B=

    0     0     1    1    0     
    0     1     1    0    0
    1     1     1    0    0
    1     1     0    0    1
    1     0     0    1    1

How do we know that the threshold is 13.

i mean, what fuction should i apply on these two matrices (A and B), so the we automatically get the answer that threshold is 13 (above 13 is '0' and below 13 is '1')?

kindly tell me the function OR something @image anylst