Rule of 720 - With Divisors
There is a clever extension to the Rule of 720 that is a bit easier to compute. Roger Taft explains and provides some examples. The KenKen solver is now capable of detecting it.
The rule is simple: If the Rule of 720 target product is not divisible by 2/3/4/5/6 then all 2s/3s/4s/5s/6s can be eliminated from each of the units that are unknown
This occurs most commonly with a 5, but cases with 3, 4, and 6 have been found.
Example 1
In the first example the block is Rows A and B.
The target product is
(720*720)/(15*240) = 144
This is not divisible by 5. So all 5s (shown in yellow) in the unknown units (boxed in blue) can be eliminated
Example 2
In the second example the target block is columns 4, 5 and 6.
The target product is
(720*720*720)/(90*12*12*72) = 400 which is not divisible by either 3 or 6. So there are many eliminations (shown in yellow) in the unknown units (boxed in blue)
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