The answer is 24.I'll tell how:

x=The total number of deer in the herd.

Half of them are grazing in the field,so x/2=The number of deer grazing.

Let's call the result of x/2 "y".Now y*3/4 has to be=9,because 3/4 of y is the number of deer playing and the rest 9 are drinking.

To find the number for y,we have to use the opposite operation for multiplication,that is,division.

So,9 divided by 3/4 is 12,and that's "y".

Now that we know that y=12,to find x we simply multiply 12 by 2 since y=x/2

So,12*2=__24 __- Ans.