Explanation:
once we know what the domain and the range of a function is, this is very simple.
the domain is the interval or set of all valid x (input) values.
the game is the interval or set of all valid y (function result) values.
so, what are all the valid x values ? that means for how many months of collection can we use this function to see how many figurines he has ?
well, we can stay with 0 (what he currently has) and go up to 5 (because after 5 months he does not want to continue this behavior).
so, the domain is
0 <= x <= 5 if we allow also fractions of months as input (if this makes sense)
or
x € {0, 1, 2, 3, 4, 5} if we allow only whole numbers.
and the range ? what are the possible result values of the function ?
for x = 0 it starts with y (or f(x)) = 100.
x = 1 gives us 100 + 3×1 = 103
x = 2 gives us 100 + 3×2 = 106
x = 3, 100 + 3×3 = 109
x = 4, 100 + 3×4 = 112
x = 5, 100 + 3×5 = 115
so, the range is
100 <= y <= 115 if we allow fractions of months
or
y = f(x) € {100, 103, 106, 109, 112, 115} if we allow only whole months as values of x.