40% is basically 2/5. This means we can let pages read be 2 units, and pages not read be 5 units.
Total pages in book = 2 + 5 = 7 units
Read pages = 2/7 of total pages
60% of the book is 3/5 of the book.
That's 3 units out of 5 units.

Now, the total number of pages is the same so the total number of units is also the same. We make them the same by changing the denominator to the same one.
The common multiple of 7 and 5 is 35
(7 x 5 = 35, 5 x 7 = 35)
Multiply both numerator and denominator by 5, 2/7 = 10/35
Multiply both numerator and denominator by 7, 3/5 = 21/35
We can now conclude that at first, 10 units of pages were read. Then after 22 more pages were read, total read pages increased to 21 units. And there are 35 units of pages in total.
21 units - 10 units = 11 units
So 11 units = 22 pages
1 unit = 22 pages ÷ 11 = 2 pages
Total pages = 35 units
= 35 x 2 pages
= 70 pages