You got it right slowpoison.. I'll try to put it in a simpler way: 1. Start both hourglasses. 2. When the 4-min hourglass runs out, turn it over (4 mins gone, 3 mins left on 7-min hourglass) 3. When the 7-min hourglass runs out, turn it over (7 mins gone, 1 min left on 4-min hourglass) 4. When the 4-min hourglass runs out (8 mins gone), turn the 7-min hourglass over (it has only been running for 1 min). When it runs out, 9 mins have elapsed. Aashish, the questions says each volume has 1000 pages. Anyway the answer is not as simple as X * 1000 pages. Imagine the ten books on a bookshelf, lined up together, left to right. | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | The answer would be 10 * 1000 = 10,000 only if the bookworm started eating from the last page of volume 1 (on the outer left) and finished eating the 1st page of volume 10 (on the outer right). The 1st page of volume 1 is in the inner left and the last page of volume 10 is in the inner right.