This is the ground plan of a group of square skyscrapers. In every row and column there is one building with 10, 20, 30 and 40 (and in larger puzzles also 50 and 60) floors but you do not know in which order. The numbers at each side indicate how many buildings can be seen in the repective row or column from this side. Higher buildings hide lower buildings. Write the height of each building into the squares on the ground plan.