IKMC 2010 - Grades 5-6 Question 26

In the grid, at least how many grey squares have to be coloured white, so that in each row and each column there is exactly one grey square?



Editable copy (G = grey, W = white):

| | | | | |
|---|---|---|---|---|
| G | W | G | G | W |
| W | G | G | W | W |
| G | W | W | G | W |
| W | W | G | W | G |
| W | G | W | W | G |