Math Is Fun Forum

JaneFairfax

Member

Registered: 2007-02-23

Posts: 6,868

Eleven

This is the latest number game invented by yours truly.

(1) Think of an integer and square it.

(2) Multiply together the number before and the number after the resulting square.

(3) If the last digit of the new number is 4 or 9, add 3; if the last digit is not 4 or 9, add 2.

(4) Multiply by 20 and subtract 29.

And the last two digits of the final number will always be!

Last edited by JaneFairfax (2009-10-26 02:29:46)

mathsyperson

Moderator

Registered: 2005-06-22

Posts: 4,900

Re: Eleven

In the third step, if the last digit is 9 you also need to add 3. (eg. Try the trick with starting number 10)

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Why did the vector cross the road?
It wanted to be normal.

JaneFairfax

Member

Registered: 2007-02-23

Posts: 6,868

Re: Eleven

Thanks! Fixed.

The point is that if your number is an odd multiple of 5, step 3 will produce a number ending in 24, while if your original number is a multiple of 10, step 3 will produce a number ending in 99. I forgot about the latter.

Last edited by JaneFairfax (2009-10-26 07:46:02)