Sheng thinks of a number, doubles it, then subtracts half the original number, then adds three.

He finishes the number with 39.

What was Sheng's original number?

2x - x/2 + 3 = 39

Let's solve this step-by-step:

Let's call the original number that Sheng thought of 'x'.

Step 1: Sheng doubles the original number.
=> 2x

Step 2: Sheng subtracts half of the original number.
=> 2x - (x/2)

Step 3: Sheng adds three.
=> 2x - (x/2) + 3

Step 4: The resulting number is 39.
=> 2x - (x/2) + 3 = 39

To find the original number, we need to solve the equation:

2x - (x/2) + 3 = 39

Simplifying the equation:

Multiplying through by 2 to eliminate the fraction:

4x - x + 6 = 78

Combining like terms:

3x + 6 = 78

Subtracting 6 from both sides:

3x = 72

Dividing both sides by 3:

x = 24

Therefore, Sheng's original number is 24.

To find Sheng's original number, we will work backwards from the given final number.

Let's start with the final number, which is 39. Sheng finished with 39, so before adding 3, he had 36.

To undo the subtraction of half the original number, we need to add half of the number back. Therefore, we add 18 (half of 36) to 36, which gives us 54.

To undo the doubling of the original number, we need to divide the number by 2. Therefore, we divide 54 by 2, which gives us 27.

Hence, Sheng's original number was 27.