A binary variable can be introduced to a mixed integer program to allow for a “threshold constraint.” A threshold constraint says that if any units are used, at least a specified minimum amount must be used. Define X as the number of students that will go on a planned field trip. The school will rent a bus only if at least 20 students plan to go on the trip. Define Y as a binary variable that equals 1 if X is nonzero, and equals 0 if X is zero (i.e., if nobody goes on the trip). If M represents a very large number, what two constraints can be added to the mixed integer program to ensure that if any students go on the field trip, at least 20 have to go?