Answer:
{0,1,2,3}
because whole numbers always starts from zero...
Data:
Humans (x) = 2 Legs
Horses (y) = 4 Legs
74 Heads
196 Legs
Solving:
Addition method

Simplify by (-4) The first equation







Now, to find the number of horses, I will use the following equation and I will replace the found value, we will have:


Number with incognito are to the left of the equality and numbers without incognito are to the right, remembering that when changing of side changes the signal.


Answer:
Humans = 50Horses = 24
Examples of this are:
2 and 
2 and 
4 and 
I'm guessing you have a work sheet going along with this which has actual images of containers. I've attached an example - if you can post the actual work sheet you're referring to I can edit this answer to correctly reflect your specific question.
25, 400 and 144 are all square numbers and 24, 300 and 145 are not.
Therefore, he must like 1600