Question 24410
Since you have to buy 1 of each animal,let's say you buy X Cats, Y Dogs and Z Mice.

Now the sum total of these animals is 100
i.e. X+Y+Z = 100
Now since each cat costs $1, X cats will cost 1x X = 1X
Similarly each dog will cost you 15Y
and Each mouse will cost 0.25Z
The sum total is $100
So 
1X+15Y + 0.25Z = 100        (1)

X+Y+Z = 100                  (2) 

Can you solve from here? Subtracting (2) from (1) will give you an equation in terms of Y and Z. Solve for either variable and put it back into any equation to get your answer.