Yes, Jim's approach is pretty much easy but a small correction in the final step ,it is 3/8 instead of 5/8

I agree with you, the way I understood is, if there is no When in the beginning of the sentence then you can keep comma after the independent clause and can continue dependent clause after comma, anyway you got a solution. good to hear

Medieval Monasteries and modern libraries

Early medieval monasteries, while clearly less accessible to outsiders, often served as repositories for texts, like modern libraries a) while clearly less accessible to outsiders, often served as repositories for texts, like modern libraries b) like modern libraries, often served as text repositori...

Ratio of Surface area of rectangular solid = sum of surface area of all the faces , so it is 2(surface area of all 3 distinct faces). Lets assume sides are of length 6,6,6 then it is a cube, Surface area of cube = 2(6*6 + 6*6 + 6*6) = 216, now, when one of it's length id doubled the sides are 6,6,12...

I think the sentence is correct because those are dependent. Sentence starts with 'when' stating that there is an action after that,so I feel it is not run-on. ( I am also beginner in GMAT so please dont mind If I am wrong and also please feel free to correct my comment)