The book is designed with "spiral learning" in mind. Vocabulary introduced in an early unit (e.g., numbers) reappears in later units (e.g., telling the time, shopping prices). This constant recycling reinforces memory and moves words from short-term to long-term memory.
Check your answers against the answer key. For every mistake you make, write the correct sentence three times in your notebook. Don't ignore your errors; study them. English Vocabulary In Use -Elementary-