Each of the following sentences has one mistake. Identify and correct the mistakes

IV. Each of the following sentences has one mistake. Identify and correct the mistakes. 

16. While we were on holiday, we spend most of our time sightseeing. 

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17. Paul listened to his iPod, so he didn’t hear the doorbell ringing. 

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18. He is a most intelligent student in our class 

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19. Your new car is more cheaper than John’s. 

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20. He left college when he is 17. 

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