When laura arrived at the library, she spent 40 min readind a book. then she spent 15 min reading a magazine. she left the library at 4: 15 pm. at what time did laura arrive at the library?

Say we minus the 15 minutes Laura spent reading the magazine from 4:15. You then get 4:00, which is the time she finished reading her book. Now if we go back 40 minutes, we can minus 60 minutes by 40 minutes which gives us 20 minutes, giving us the time of 3:20 pm which is when she arrived. Let me know if you have any more questions!