there are d horses in a field there are 16 more female horses than male horses of the horses in the field only 25% of the male horses are brown. How many horses are black.

Use f and m for "female" and "male."  Then f = m + 16, and f + m = d (total)

So now we have the system of linear equations

f = m + 16
f + m = d

We can eliminate f by subst. m + 16 for f in the second equation:
                                                                                                           d-16
m + 16 + m = d, or 2m + 16 = d.  Find m.  2m = d - 16, so that m = --------
                                                                                                               2

Assuming that the male horses are either brown or black, and that 25% of these are brown, then the rest must be black.  This number is
           
            d-16
0.75 [ --------- ] .
               2