Answer:
3. (a) x, y ∈ W
5. (b) x, y ∈ W
Explanation:
Given two problems in which x and y represent numbers of seats, you want to know what, if any, domain the variables are restricted to.
Counting numbers
A number of seats is a counting number, possibly including zero. Generally, it will be a member of the set of whole numbers: {0, 1, 2, 3, ...}. They will not be negative, and will not have a fractional part: x ≠ -1, x ≠ 2.3.
In both your problems, x and y are defined as numbers of seats, so will be from the domain of whole numbers.
x ∈ W, y ∈ W
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Additional comment
The various named domains are usually represented by ...
ℝ — real numbers
ℕ — natural numbers {1, 2, 3, ...}
ℚ — rational numbers
ℤ — integers
ℂ — complex numbers
W — whole numbers (Natural numbers and zero)
We're unfamiliar with the use of I, which we presume is intended to represent Integers.