The probability that at least one twin will go on the trip is 1−18/20×19/19=19/20.
In a mathematics class with a total of twenty students, the teacher randomly selects two students for a trip. To calculate the probability that at least one twin (Jenny or Penny) is chosen, we consider the complementary probability of neither twin being selected. The probability of the first student not being a twin is 18/20 (since there are 18 non-twin students out of 20), and the probability of the second student not being a twin is 19/19 (as only one student has been chosen, leaving 19 students for the second pick).
Multiplying these probabilities gives the chance that neither twin is selected. Subtracting this value from 1 provides the probability that at least one twin is chosen, yielding 19/20. This calculation accounts for the possibility that either Jenny or Penny, or both, are among the students selected for the trip.