Consider the following trick:
Step1: Think of any number.
Step2: Subtract the number you have thought with 1.
Step3: Multiply the result with 3.
Step4: Add 12 with the result.
Step5: Divide the answer by 3.
Step6: Add 5 with the answer.
Step7: Take away the number you have thought from the answer, that is, subtract the answer from the number you have thought.
Answer: 8
Why?
Let's take the number thought as X and try the above steps on it:
Step1: Think of any number -> X
Step2: Subtract the number you have thought with 1 -> X - 1
Step3: Multiply the result with 3 -> (X - 1) * 3
Step4: Add 12 with the result -> [(X - 1) * 3] + 12 = 3X - 3 + 12 = 3X + 9
Step5: Divide the answer by 3 -> (3X + 9) / 3 = 3X/3 + 9/3 = X + 3
Step6: Add 5 with the answer -> (X + 3) + 5 = X + 8
Step7: Take away the number you have thought from the answer, that is, subtract the answer from the number you have thought -> (X + 8) - X = 8
Now try this one:
Step1: Think of any number.
Step2: Multiply the number you have thought with 3.
Step3: Add 45 with the result.
Step4: Double the result.
Step5: Divide the answer by 6.
Step6: Take away the number you have thought from the answer, that is, subtract the answer from the number you have thought.
Answer: 15
In order to create a trick play, you can make up its steps in a specified order and then present it in the reverse order.
Example: Take X as what would be thought and Y as the answer.
Write Y as the sum of two numbers, Y = A + B; for example, if your answer is 9 then you may choose 9 = 4 + 5, 9 = 14 + (-5), 9 = 8 + 1, etc.).
X + Y = X + Y
X + (A+B) = X + Y
(X + A) + B = X + Y
Choose an arbitrary number, C, e.g. C = 4.
(C/C) * [(X + A) + B] = (C/C) * (X + Y)
(CX + CA) / C + B = X + Y
Take D = CA.
Now make the steps by reversing the above operations. As an example (very simple one):
Y = 10, A = 6, B = 10-6 = 4, C = 2, D = 2*6 = 12
Step1: Think of any number.
Step2: Multiply the number you have thought with 2 (rem. C = 2).
Step3: Add 12 with the result (rem. D = A*C).
Step4: Divide the answer by 2.
Step5: Add 4 with the result (rem. B = 4).
Step6: Take away the number you have thought from the answer.
Answer: 10 (rem. Y = 10).
Now, you can try to make some complex ones.