Mr.A reads 3/7 of a book on one day and 3/4 of the remaining part on another day. If there were 45 pages unread, how many pages did book contain?

Let Book contain=X Pages
A read 3/7 of X=(3/7)*X=3X/7
Remaining Pages=X-3X/7=(7X-3X)/7=4X/7
A read remaining Pages=3/4 of 4X/7=3/4*4X/7=12X/28
A left Pages=4X/7-12X/28=(16X-12X)/28=4X/28=X/7
Now X/7=45=
X=45*7=315