in a group of 120 students ,68 had ridden a bus, 78 had ridden a LRT, 33 had ridden the jeep, while 40 had ridden both bus and LRT, 20 had ridden the bus and the jeep, 19 had ridden LRT and jeep, 15 had ridden the bus the LRT and the jeep. how many did not ride on any of the three modes of transportation?​