Use the rule that states any number times zero is zero when you create this number trick. Understand that at some point you need to multiply the number times 0.
Tell your friend that you can divide, multiply, add and subtract numbers in your head as fast as a calculator and can come up with the final answer to a long equation.
Suck them in to your scheme by telling them they can even pick some of the numbers. Tell them to write the numbers down to put in a calculator later and find if the final answer is correct.
Start with telling them to give an equation that has a four-digit number divided by another four-digit number. Tell them that you then multiply the quotient by a four-digit number you select. They can shout out a four-digit number to add to the product. You then state a four-digit number to subtract from the sum.
Explain that the difference is used as the first number on the next sequence. They give a three-digit number to divide into it. You give one to multiply, they choose one to add to the product and finally you choose a number to subtract from that.
Explain that they can then divide the sum by any two-digit number. You give a two-digit number to multiply the quotient, they give a two-digit number to add, and you give the two-digit number to subtract. The final step is to use that answer in the same way but divide, multiply, add and subtract single-digit numbers.
Shout out the answer quickly. You didn't really do any of the steps in the equation except the last three. When it was your turn to use a one-digit number to multiply, you used zero. Any number times zero is zero. When they added a single-digit number to zero, such as 6, the answer is that number, or 6. All you had to do is subtract any number from 6.