In Fahrenheit 451, on what page did Mildred tell Guy that Clarisse was dead?

Of course, there are many different versions and editions of this book in print, so it is very hard to give you the right page number.

In one edition of the book (sold on Amazon), the page you are looking for is page 47.  In that book, Part 1 starts on page 1 and Part 2 starts on page 69.  So the page you are looking for is about two thirds of the way through Part 1.

Another way to find it is to know that it is just a bit before the part where Guy gets sick.  It happens as they are going to bed and then in the morning he's sick.