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Nine digits => two numbers => largest product



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in general the largest number produced by multiplication is achieved when the two numbers are close together.

so we want a 4 digit and a 5 digit number.

a b c d e 

* f g h i



the largest first digit occurs when a and f are 9 and 8. since we want them to be close, its better for a to be 8 and f to be 9.

the second largest digit will be 2 + 9*b +8*g. the largest value occurs when b is 7 and g is 6. 

this puts the max value at 8300000 aprox. the next largest digit is 7*6 +9*c +8*h. the max value occurs when h is 4 and c is 5.

you can continue. the final answer is: 9642 *87531.

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Toying with a script to spit out the answers quickly, it seems this is the general pattern:


If the digits are dm..dn (contiguous) the answer looks like:


    d n dn-3dn-5.. . * dn-1dn-2dn-4...


regardless of m and n. This works regardless of whether there is an even or odd number of digits.


Nice!  post-1048-0-84137600-1395017309.gif

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