How would you solve this? Java coding for multiplication.

**fatabass** · 12-23-2011, 08:25 PM

*** ( Some number with 3 digits )
** ( Some number with 2 digits )
x
-------------
****
****
-----------
*****

where all the asterisks are prime digits either: 2,3,5 or 7

Ideas, approaches, or even code mayde?

**JosAH** · 12-23-2011, 08:39 PM

It cannot be solved because the righrmost digit in the fourth row has to be a zero.

kind regards,

Jos

**fatabass** · 12-23-2011, 08:41 PM

What makes you think that?

I am sure it is a "can be solved" question.

**pbrockway2** · 12-23-2011, 08:59 PM

Originally Posted by JosAH

righrmost

@OP: Can you post again using code tags, because it makes a difference. Put [code] at the start of the calculation and [/code] at the end. That way the formatting will be preserved and we can all see what is supposed to line up with what.

**fatabass** · 12-23-2011, 09:02 PM

**JosAH** · 12-23-2011, 09:08 PM

The last digit of the top two numbers have to be a 3 and a 5 (...3x..5 = ....5 (or ..5x.3 = ....5 is the only possibility), so the last digit of the result is 5 as well. Maybe consistent reasoning like this can help you to solve this puzzle. If there are too many alternatives I'd take the lazy approach (brute force)

kind regards,

Jos

**fatabass** · 12-23-2011, 09:11 PM

I am trying to solve it with Java :)
Thanks though... :)

I have this idea of looping for all numbers in the first 2 rows and multiplying them and to check if they are all prime numbers(2,3,5,7)..

But can there be a better approach ?

**pbrockway2** · 12-23-2011, 09:15 PM

Totally slow answer adding nothing removed.

@OP: That's more or less what I would do.

**JosAH** · 12-23-2011, 09:30 PM

There are four different digits (2, 3, 5 and 7) so you should count in base 4. There are 4x4x4 possible first numbers and 4x4 possible second numbers. You can try them all after mapping 0 --> 2, 1 --> 3, 2 --> 5 and 3 --> 7 and see which numbers have the correct result.

kind regards,

Jos