Dependent events are refered to as the events that occur when occurrence of one of the events has an effect on the chances of the other event occuring as well.
For a dependent event, we should note that the outcome of the second event affects the dependent event. In a case when it doesn't affect each other, it's called independent event.
The free body diagram of book y indicates: a weight of 5 N from book z, and a reaction of 5 N on book z; a weight of 10 N acting on book x, and a reaction of 10 N from book x
The net downward force on book y is 5 + 10 - 10 = 5 n.