At a college party 5 girls are sitting in a row. P is to the left of M and to the right of O. R is sitting to the right of N, but to the left of O. Who is sitting in the middle?
The weights of 4 boxes are 20, 30, 40 and 60 kilograms. Which of the following cannot be the total weight, in kilograms, of any combination of these boxes and in a combination a box can be used only once?