Iread 1/4 of a book on saturday and 2/5 of the remainder on sunday. if 36 pages still remain to be read, how many pages are there in this book altoghether?

If all pages = x
on saturday = 1/4 x
on sunday = 2/5 * (x-1/4 x) = 2/5 * 3/4 x = 3/10 x
then remaining pages = 36 = x-1/4 x - 3/10 x

(multiplied by 20)

720= 20x-5x-6x
720= 9x
so x = 80

There are 80 pages in this book altogether.