A driver sets out on a journey. For the first half of the distance she drives at the leisurely pace of 20 mi/h; she drives the second half at 60 mi/h. What is her average speed on this trip?.

At work claire has a deadline that is rapidly approaching as she works she finds that she is doing more work in a shorter amount of time than usual she thinks more quickly and her senses and memory are sharper what is responsible for claire working more efficiently