Question

A girl can finish a book in 25 days if she reads 18 pages of it every day. How many days will she take to finish it, if she reads 15 pages every day?

Answer 1

Days taken to finish the book if 18 pages are read everyday = 25
Days taken to finish the book if 1 page is read everyday = 18 $\times$25 [less pages, more days]
Now, days taken to finish the book if 15 pages are read everyday = $\frac{18\times 25}{15}$ = 30 [more pages, less days]
Hence, the girl will take 30 days to finish the book if she reads 15 pages everyday.