If 5 students utilise 18 pens in 9 days, how long at the same rate, will 66 pens last for 15 students?

11 12 13 14

11

According to the question: \(\frac{\text{5 x 9} }{18 }\) = \(\frac{\text{ Days x 15} }{66 }\) \(\frac{\text{45} }{18 }\) = \(\frac{\text{ Days x 15} }{66 }\) Days = 11  Thus, the answer is 11 days.