I have multiplied all odd integers below 15.

The penultimate digit of the 6-digit resulting number is 3.

The above hint is sufficient for restoring the correct result without reverting to my multiplication.

Go ahead, replace the asterisks by digits: N=****3* !

In retrospect I could have done this without the 3 because I know 7*11*13 = 1001 so I just had to do 3*5*9 in my head.

3*5*9 = 135  so  1*3*5*7*9*11*13*15 = 135135

What I actually did was fill in the 5 at the end.  Then the 1001 occurred to me so I wrote _35_35 and figured the missing digits had to match and make the digit sum divisible by 9.  So they are both 1's.

Posted by Jer on 2013-10-08 13:57:09