Solution:

Happy Birthday To You

Explanation:
Happy + Bird(-d) + sunday(-sun) + to(2) + you(u)

Solution:

If we remove the first letter of the word and place it in the end. It will be read the same from front and back.

Solution:

3 + 3 = 8 can be set true by moved to one of two solution.
A) 5 + 3 = 8
B) 3 + 5 = 8

Solution:

A Spider

Solution:

Notable
Not-able, &
No-table.

Complete Sentence:
A notable Surgeon was not able to perform surgery on a lady as there was no table.

Solution:

Horse = 10
Cowboy-Boot = 1
HorseShoe= 2
The Last equation = 21

Detailed Explanation :
Equation1:
3Horse = 30
=> Horse = 10 .....(1)

Equation2:
1Horse + 2HorseShoe + 2HorseShoe = 18
10 + 4HorseShoe =18
HorseShoe = 2 .....(3)

Equation3:
2HorseShoe - 2Cowboy-Boot = 2
4 - 2Cowboy-Boot = 2
=> Cowboy-Boot = 1 ... (2)

Equation4:
Cowboy-Boot+Horse*HorseShoe = 1+10*2
=>21

Solution:

10

Explanation:
Equation 1 + 9 + 11 = 1 can be derived from
One (o) + nine (n) + eleven (e) = one => 1

Similarly for equation,
12 + 11 + 9
Twelve (t) + eleven (e) + nine (n) => ten (10)

Solution:

20

Explanation :
The sequence is given in the puzzle is :
11 15 19 12 16 ? 13 17 21 14 18 22

Let's break the given series as below:
11 15 19
12 16 ?
13 17 21
14 18 22

Now read the number from left hand side from top to bottom as :
So the number that will replace '?' is 20.

Solution:

The solution is provided in the image below.

Solution:

In the attached picture, you can see that the formation allows 2 castles to be protected by the walls. Congrats!