Answer is 20.
Explanation:
Let’s say he has x dollars with him.
1. On cake, he spends its half plus 2, so he is left with: x-(x/2+2) = x/2-2

2. On balloons and streamers, he spends half of the left plus 2
Left: (x/2-2) -(x/4-1+2)= x/4 -3.

3. On candy, half the left plus 1
Left: (x/4-3) -(x/8-3/2+1)= 0
Solving, x=20.

Without any approximation, the results turn out to be:
Jasmine ~4.7
Thibault~ 4.7
Noah ~ 1.5

Now, considering that Jasmine and Thibault had equal slices, they must pay equal. Thus they are rounded off to 5 and Noah pays 1. This makes sense also because the decimal value of Noah is least so he should be the one to pay lower value of the rounded numbers.

The third room as the lions cannot survive 3 years without eating.