# Nested sudoku in entrance exam

My university set this question in 2013's entrance exam. Also the red boxes include from 1 to 9. What's the solution for this sudoku?

• i would very much like to know which university this is. :P Jun 4, 2017 at 18:31
• @smci why 2x2? There is no 2x2 constraint Jun 5, 2017 at 2:44
• Counter-intuitively, the red boxes actually make it easier rather than harder Jun 5, 2017 at 2:45
• @thecoder16 It's Keio university in Japan. Jun 5, 2017 at 11:00
• @smci these are commonly known as windoku or hyper sudoku. Jun 5, 2017 at 22:36

## Step-by-step explanation

• In the upper right red square, there's only one possible position for 7.
• In the lower right red square, there's only one possible position for 5.
• In the seventh row, there's only one possible position for 7.
• In the eighth row, there's only one possible position for 6.
• In the second row, there's only one possible position for 3.
• In the eighth column, there's only one possible position for 3.
• In the bottom middle 3x3 box, there's only one possible position for 3.
• In the seventh row, there's only one possible position for 5, then only one for 4, then only 3 is left.
• In the lower right red square, there's only one possible position for 2, then only 1 is left.
• Now only 1 is left in the bottom right 3x3 box.
• In the eighth row, there's only one possible position for 7, then only one for 8, then only 4 is left.
• Now only 2 is left in the bottom left 3x3 box.
• In the first column, there's only one possible position for 5, then only one for 1, then only 6 is left.
• In the top left 3x3 box, there's only one possible position for 7.
• In the lower left red square, there's only one possible position for 6, then only 4 is left.
• In the second column, there's only one possible position for 1, then only one for 2, then only 5 is left.
• In the middle left 3x3 box, there's only one possible position for 3, then only 6 is left.
• In the sixth row, there's only one possible position for 2, then only 8 is left.
• In the ninth column, there's only one possible position for 7, then only 8 is left.
• In the fifth row, there's only one possible position for 6.
• In the seventh column, there's only one possible position for 1, then only 8 is left.
• Now only 2 is left in the top right 3x3 box.
• In the upper left red square, there's only one possible position for 8, then only one for 5, then only 4 is left.
• Now only 4 is left in the third row.
• In the upper right red square, there's only one possible position for 2, then only one for 1, then only 3 is left.
• Now only 4 is left in the fourth row and in the eighth column.
• Now only 1 is left in the fifth row, and only 7 in the second row.

And from there it's easy to get the final solution.