15. Margot wants to prepare a jogging timetable. She wants to jog exactly twice a week, and on the same days every week. She does not want to jog on two consecutive days. How many different timetables could Margot prepare?
A 18
B 16
С 14
D 12
E 10
15. C Whichever day of the week Margot chooses for her first jogging day, there are four other days she can choose for her second day since she does not want to jog on either the day before or the day after her first chosen day. Therefore there are $7 \times 4=28$ ordered choices of days. However, the order of days does not matter when forming the timetable, only the two days chosen. Hence Margot can prepare $28 \div 2=14$ different timetables.