Here are the mistakes in each sentence:

47. **Mistake:** "wonderfully" should be "wonderful." The correct phrase is "I found it wonderful to travel abroad."

48. **No mistake** in this sentence; it is correct.

49. **Mistake:** "laziness" should be "laziness." The phrase can be improved to "He failed the exams because of his laziness."

50. **Mistake:** "had driven" is incorrect; it should be "drove." The correct phrase is "The policeman drove ten minutes ago."

So, the errors are in sentences 47, 49, and 50.