Susanna read 1/5th pages of a book. if she reads further 40 pages, she would have read 7/10th pages of the book, how many pages are left to be read

So we can take X as the n.o of pages of the entire book
so 1x/5 +40= 7x/10
pls remember the total n.o of each book is X
therefore
40=7x/10-1x/5
to solve this u need to make the denominator's the same
40=7x/10-2x/10
40=5x/10
40*10=5x
400=5x
400/5=X
80=x