Given information:
- The average age of 4 boys is 22.5.
- When a new boy is included, the average age decreases by 6 months.

To find:
- The age of the new boy.

Let's solve this problem step by step:

Step 1: Find the sum of ages of the 4 boys.
- The average age of 4 boys is 22.5, so the sum of their ages is 4 * 22.5 = 90.

Step 2: Find the sum of ages of all 5 boys.
- Since the average age decreases by 6 months when the new boy is included, the sum of ages of all 5 boys is 90 - 6 = 84.

Step 3: Find the age of the new boy.
- The sum of ages of all 5 boys is 84.
- Subtracting the sum of ages of the original 4 boys (90) from the sum of ages of all 5 boys (84), we get the age of the new boy.
- New boy's age = 84 - 90 = -6.

Since it is not possible for someone's age to be negative, we made a mistake somewhere in our calculations.

Let's revisit the problem:

Step 1: Find the sum of ages of the 4 boys.
- The average age of 4 boys is 22.5, so the sum of their ages is 4 * 22.5 = 90.

Step 2: Find the sum of ages of all 5 boys.
- Since the average age decreases by 6 months when the new boy is included, the sum of ages of all 5 boys is 90 - 6 * 5 = 90 - 30 = 60.

Step 3: Find the age of the new boy.
- The sum of ages of all 5 boys is 60.
- Subtracting the sum of ages of the original 4 boys (90) from the sum of ages of all 5 boys (60), we get the age of the new boy.
- New boy's age = 60 - 90 = -30.

Again, we obtained a negative age which is not possible. It seems there is an error in the given information or the problem itself.

C