Answer: There are 
ways of doing this
Hi!
To solve this problem we can think in term of binary numbers. Let's start with an example:
n=5, A = {1, 2 ,3}, B = {4,5}
We can think of A as 11100, number 1 meaning "this element is in A" and number 0 meaning "this element is not in A"
And we can think of B as 00011.
Thinking like this, the empty set is 00000, and [n] =11111 (this is the case A=empty set, B=[n])
This representation is a 5 digit binary number. There are 
of these numbers. Each one of this is a possible selection of A and B. But there are repetitions: 11100 is the same selection as 00011. So we have to divide by two. The total number of ways of selecting A and B is the 
.
This can be easily generalized to n bits.
(0,-3)
Was the two supposed to be on the top
Group 'em together
a
b
−
a
+
1
−
b
a
b
−
a
=
a
(
b
−
1
)
Notice that there will be a 1 as without it it'll simply be ab
1
−
b
=
1
(
1
−
b
)
Notice that it doesn't match with the upper one... so we'll change the signs
1
(
1
−
b
)
=
−
1
(
b−
1
)
(try to multiply them now!!
Jot them down in one expression
a
(
b
−
1
)
−
1
(
b
−
1
)
You get!!!!!!
(
a
−
1
)
(
b
−
1
)
B. Integers
C. Whole Numbers
D. Rational Numbers
E. Natural Numbers
