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Hi there.

I am looking for a mathematical number force with the property that knowing the last operation, would allow you to figure out the original number.

for example the multiples of nine force;
Think of a 2 digit number, and add them together. (28. 2+8=10)
Subtract the new number from the original 2 digit number. (28-10 = 18)

now if i knew the last operation was '-10' i can work out the original number was 28, by simply adding the 10 back on.

I am looking for a force that does something similar, but with the same outcome for each possible number.
I don't mind if i need a calculator to be able to figure it out.

thanks!

Think of a two digit number.

Add the digits together and take that number away from the first number you thought of.

Add those digits together.

Add the number you first thought of to the number you have just "randomly come up with".

The final total is their original number plus 9.

Or have I misunderstood your request?

If I am honest, I am always a bit funny about anything involving maths for two reasons. One, people get even simple maths wrong, especially when under pressure, like helping out with a trick. When you have things like add a number, you can check, or get someone else to check the working. Second, stuff like this is very easy to work out for anyone with half an interest in maths and a moderately decent memory.

Having said that, we have a couple of real maths bores, um, experts on this forum and maybe they can give you something really clever.

Part-Timer wrote:
If I am honest, I am always a bit funny about anything involving maths for two reasons. One, people get even simple maths wrong, especially when under pressure, like helping out with a trick. When you have things like add a number, you can check, or get someone else to check the working. Second, stuff like this is very easy to work out for anyone with half an interest in maths and a moderately decent memory.

Having said that, we have a couple of real maths bores, um, experts on this forum and maybe they can give you something really clever.

I read somewhere that Math was one of the biggest fears people have. I know if I am at a magic show and the magician starts something with numbers my mind drifts. And if they ask me to do something with numbers the effect simply isn't going to work. I suck at math and I'm dyslexic, and if I'm handed a calculator then I worry the whole time of looking like an idiot even with calculator in hand.

And if it is the magician doing the math I'm always suspect and I think the rest of the audience also becomes suspect that it is just a clever numbers thing.

i agree, whilst i enjoy maths and numbers, i realise that I'm in the minority.
Even with calculators etc it could easily go wrong too.
Wouldnt it be easier to force a number instead? there are lots of methods for that, including writing numbers on a blank deck of cards.

Relish wrote:Even with calculators etc it could easily go wrong too.

In the last 6 weeks or so, I have seen several effects involving calculators go wrong. One spectator I saw multiplied 5 4-digit numbers together and ended up with a 3-digit number as the result

If you do go down the calculator route, some pointers...

1. Try to choose someone who is familiar with using a calculator to use the calculator - it's not safe to assume that someone with a calculator on their phone will know how to use it.
2. Perhaps key in the first number yourself (beside the volunteer to keep it fair) to illustrate what to do, and make it harder for them to "reset"
3. Make your instructions as clear as you can to avoid misinterpretation (and make sure your volunteer can understand the instructions - avoid language difficulties).
4. Know how to set/reset mid-routine.
5. Have an out.

I have also seen two performers - one is a well known name and the other a 15-year-old - skilfully dealing with a calculator-based hiccup using a combination of 4 and 5 above. These routines did not go wrong, they just had an extra couple of bits in them.

Ian

I do a chair routine which eventually generates three two digit numbers which a volunteer has to add. The task is easier than falling off a log but I always offer a calculator and also suggest they get the person sitting next to them to check and agree the total. Even so, on one occasion two people and a calculator managed to get the total wrong!