Observe the number sequence, 22, 21, 23, 22, 24, 23, ... what number should come next?

For Domain 2, Co-domain is 4. For Domain 3, Co-domain is 6.  
The pattern is Co-domain $= 2 \times \text{Domain}$.

For Domain 1, Co-domain $= 2 \times 1 = 2$.  
For Domain 4, Co-domain $= 2 \times 4 = 8$.  
The completed table a is:  
Domain: 1, 2, 3, 4  
Co-domain: 2, 4, 6, 8

For Domain 20, Co-domain is 200. For Domain 40, Co-domain is 400.  
The pattern is Co-domain $= 10 \times \text{Domain}$.

For Domain 10, Co-domain $= 10 \times 10 = 100$.  
For Domain 30, Co-domain $= 10 \times 30 = 300$.  
The completed table b is:  
Domain: 10, 20, 30, 40  
Co-domain: 100, 200, 300, 400

**i)** What are the missing numbers in the co-domains in table a above?  
The missing numbers are $\boxed{2 \text{ and } 8}$.

**ii)** Write down the set of elements in the co-domain of the relation in table b.  
The set of elements in the co-domain of table b is $\boxed{\{100, 200, 300, 400\}}$.

Let P be the domain and Q be the co-domain.  
Given pairs: $(3, 6)$, $(4, 10)$, $(?, 15)$, $(?, 28)$.  
Observe the differences in Q values for consecutive P values:  
$10 - 6 = 4$ (for P=4 and P=3)  
$15 - 10 = 5$ (for P=? and P=4)  
This suggests the differences are increasing by 1. This is a sequence of triangular numbers, where $Q = \frac{P(P+1)}{2}$.

For $Q=3$:
$$
3 = \frac{P(P+1)}{2}
$$
$$
6 = P^2 + P
$$
$$
P^2 + P - 6 = 0
$$
$$
(P+3)(P-2) = 0
$$
Since P must be positive, $P=2$.  
For $Q=15$:
$$
15 = \frac{P(P+1)}{2}
$$
$$
30 = P^2 + P
$$
$$
P^2 + P - 30 = 0
$$
$$
(P+6)(P-5) = 0
$$
Since P must be positive, $P=5$.  
For $Q=28$:
$$
28 = \frac{P(P+1)}{2}
$$
$$
56 = P^2 + P
$$
$$
P^2 + P - 56 = 0
$$
$$
(P+8)(P-7) = 0
$$
Since P must be positive, $P=7$.

For $P=1$:
$$
Q = \frac{1(1+1)}{2} = \frac{2}{2} = 1
$$
For $P=6$:
$$
Q = \frac{6(6+1)}{2} = \frac{42}{2} = 21
$$
The completed table c is:  
P: 1, 2, 3, 4, 5, 6, 7  
Q: 1, 3, 6, 10, 15, 21, 28

**i)** Write down the set of elements in the domain in table c above.  
The set of elements in the domain of table c is $\boxed{\{1, 2, 3, 4, 5, 6, 7\}}$.