Let the total number of deers be x.
No. of deers in the forest = 1/4x
No. of deers in the field = 1/3x
No. of deers drinking water = 15
Total number of deers = 1/4x + 1/3x + 15 = x
3/12x + 4/12x - x = -15
7/12x - 12x/12 = -15
-5x/12 = -15
5x/12 = 15 (both minuses gets cancelled)
5x = 15 * 12
5x = 180
x = 180/5
The total number of deers are 36.