While since Rajeev is already below the surface by 12.6 ft it’ll be negative (-12.6) and since he went down another 8.7 ft it’ll just be minus since he is still going under, so -12.6-8.7 will be your answer. :)
First you have all ten books, but then every time after, you loose one because you can't put the same book in the group twice.
10 * 9 * 8 * 7 = 5040 groups
