When looking through the documentation for a specific class, you never actually see the source code for that class. Instead, you see an interface for that class with descriptions of what all of the constructors and methods do. This is a real-world example of what concept

Acomputer has a two-level cache. suppose that 60% of the memory references hit on the first level cache, 35% hit on the second level, and 5% miss. the access times are 5 nsec, 15 nsec, and 60 nsec, respectively, where the times for the level 2 cache and memory start counting at the moment it is known that they are needed (e.g., a level 2 cache access does not even start until the level 1 cache miss occurs). what is the average access time?