Question

Suppose that A={5,8}, B={7,8}, C={7,8,9}, and D={5,7,8}. Determine which of these sets are subsets of which other of these sets.

A⊆
A
B
C
D

B⊆
A
B
C
D

C⊆
A
B
C
D

D⊆
A
B
C
D

Answer 1

every set is a subset of itself

Answer 2

also a set X is a subset of a set Y if every element in X is also in Y

Answer 3

For example:
if X={1,2,3,4,5} and if Y={1,2,3,4,5,6}, then X is a subset of Y since every element in X is also in Y.
Another example:
If X={1,2,3} and Y={1,2,4}, then X is not a subset of Y since every element of X is not included in Y.