Used to this at school, if i'm right, the answer is always half of the number you add on.
The way we did it, was think of a number, any number, double it, add on, eg 50, half the total,, now take away the number you thought of, and the answer should be 25
you can use any permutation, initial number, and any sytsem, provided you keep the sequence, the answer is always half the added on.
Hi John, sorry, That took me about 30mis to post. being pulled about by my daughter, then I re-read your post, I hadn't noticed your second add on, and on mine, also forgot a subtraction.