Correct the sentences with mistakes.

1. I thought very hard but couldn't remember his name. (correct) 2. Was he able to talk to his boss? (correct) 3. I couldn't do the sum myself and asked father to help me. (correct) 4. They could not play their parts so well that the audience roared with laughter. (correct) 5. How did you manage to come so early? (correct) 6. I could answer all the teacher's questions and got five. (correct) 7. The children couldn't go to the beach as it was raining. (correct) 8. Sally managed to win the 100-meter race. (correct) 9. At last, we could see the problem more clearly. (correct) 10. Couldn't you explain the problem more clearly? (correct)