The rules of KenKen are as follows:

* You must fill in the numbers 1 through 6, one number per box, in the grid, so each row and each column contains each number exactly once.
* Each heavily outlined section of the grid is marked for you with a number and a symbol of an arithmetic operation. It indicates that the numbers to be placed in the boxes in that section, when combined via that operation, yield the given number. For example, if it says "60×", then the numbers in the boxes in that section must multiply to sixty. For division ("/") and subtraction ("−") there will be precisely two boxes in the section: one of them divided by (or subtracted from) the other must yield the given number.

The following puzzle is taken directly from Simon Tatham's collection (where he calls the puzzle type "Keen"). It's one that I found unusually challenging, which is why I'm sharing it with you.

[![a six-by-six KenKen puzzle: row 1 column 1, 2/, including the box to the right; row 1 column 3, 9+, including the box to the right; row 1 column 5, 1−, including the box to the right; row 2 column 1, 60×, including the two boxes below; row 2 column 2, 2/, including the box below; row 2 column 3, 90×, including the box to the right and the box below the latter; row 2 column 5, 3/, including the box below; row 2 column 6, 8+, including the box below; row 3 column 3, 2/, including the box below; row 4 column 2, 1−, including the box below; row 4 column 4, 1−, including the box below; row 4 column 5, 1−, including the box below; row 4 column 6, 7+, including the box below; row 5 column 1, 12×, including the box below and the box to the right of the latter; row 6 column 4, 60×, including the two boxes to the right](https://i.stack.imgur.com/UyqYU.png)](https://i.stack.imgur.com/UyqYU.png)