Find and correct the mistake in each sentence: 1. Luis can’t have worked very hard. He was promoted to manager! 2. Your assistant mustn’t have been more helpful; you’re lucky to…

Here are the corrections with brief explanations:

• 1. Original: Luis can’t have worked very hard. He was promoted to manager! Correction: Luis must have worked very hard. He was promoted to manager! Explanation: A promotion suggests he did work hard; can’t have = impossible in the past, which contradicts the result.
• 2. Original: Your assistant mustn’t have been more helpful; you’re lucky to have her. Correction: Your assistant couldn't have been more helpful; you're lucky to have her. Explanation: Couldn't have been more helpful expresses the strongest possible past activity (extremely helpful); mustn't have is not natural here.