You think of a starting number. For example, 45.
When you add 3, you have your original number plus 3. In this case, 48.
When you double it, you have twice your original number plus 6. In this case, 96.
Then you add 4, and now you have twice the original number plus 10. In this case, 100.
Then you halve your number, and you will have your original number plus 5. In this case, 50.
Finally, when you subtract the original number, you will have:
Your original number - Your original number + 5 ( Which is obviously, 5! )
This is always true whatever number you chose at the beginning because at the end, you’re subtracting your original number by your original number. And since whenever you substract a number by itself, you always get 0. For example, if we use 99 as the original number, we will still end up with:
Your original number - Your original number + 5
= 0+5
= 5.