BdMO National Primary 2015#5

[samiul_samin]

Surjo is four years old and he is learning to write numbers. His math notebook looks like a square grid with $20$ rows and $20$ columns. He usually writes the numbers from top to bottom and when one column is finished he starts writing along the next column. One day he starts writing the numbers from left to right (along the rows). How many of the numbers will be placed in exactly the same place where it would have appeared if he had written along the columns?

[samiul_samin]

If we think the grid as a cartesian co ordinate and the points as $(x,y)$ then
According to the question the given condition is only possible if $x=y$
So,answer is $\fbox {20}$ .