Solution: The number of ways to choose 3 18th-century manuscripts is $inom53$. Total ways to choose 3 manuscripts from 15 is $inom153$. The probability is $rac{inom53}{inom153} = rac10455 = rac291$. $oxed{\dfrac291}$Question: If the combined population of two marine mammal species in a region is 10 and the sum of their squared populations is 50, find the sum of their cubes.