A boy read 3/4 th of the book on one day and 4/5th of the remainder on aonther day. If there were 30 pages unread: How many pages did contain ?

Let the total no of pages be x
And the three fourth of the book is 3/4 of x = 3x/4
He read 3/4^th of the book, so the pages left = x- 3x/4 = x/4
So the remainder = x/4
He read 4/5^th of the remainder on the next day so 4/5^th  of x/4 = x/5
Since 30 pages are unread
so we have
x-(3x/4) - (x/5) = 30
$$\begin{gathered} \frac{x}{1}-\frac{3x}{4}-\frac{x}{5}=30 \\ or taking LCM of 4,5,1 we get 20, so \\ \frac{20x-15x-4x}{20}=30 \\ \frac{x}{20}=30 \\ x=600 \\ So the number of pages is 600 pages. \end{gathered}$$