LetXandYbe the amounts of time (in minutes) that a particular commuter must wait for a train on two independently selected days. Assume thatXandYhave a uniform distribution. Define a new random variableWbyW=X+Yas the sum of the two waiting times. The set of possible values forWis the interval from 0 to 40 (because bothXandYcan range from 0 to 20). It can be shown that the density curve ofWis as pictured. (This curve is called a triangular distribution, for obvious reasons!)

Density 0.05 W 20 40 Minutes