S is a subset within a universal set, U.
If S = {x, y, 4, 9, ?}, which could describe U?
O
O
O
O
U = {keys on a keyboard;
U = {letters
U = {numbers)
U = {punctuation marks}

U = {keys on a keyboard}, choice 1

We can rule out choice 2 since {4,9,?} are not letters.

We can rule out choice 3 since {x,y,?} are not numbers.

We can rule out choice 4 since {x,y,4,9} are not punctuation marks.  

true statements:

a) x + y = y + x   ⇒ commutative property of addition

b) (x * y) * z = x * (y * z) ⇒ associative property of multiplication

d) (x + y) + z = x + (y + z) ⇒ associative property of addition

false statements:

c) x - y = y - x   should be: x - y = - y + x

e) (x - y) - z = x - (y - z) should be: (x - y) - z = x - (y + z)

step-by-step explanation:

answer is more than one

step-by-step explanation: