If it took Sean 1 1/2 to read 1/5 of the book, it took Sean 3 hours to read 2/5 of the book. Now we need to figure out how long it will take Sean to read 2.5/5 of the book, which is equivalent to 1/2 of the book. Since half of 1.5 is 0.75 or 45 minutes, we must add that to our 3 hours. This adds up to 3 hours and 45 minutes, which is the same thing as 3 45/60, which is the same thing as 3 3/4 hours.