What is the hit and miss rate of the cache? Basically you need to figure out the miss rate for some chunk of data or packet and then look at the correct multiple for your given file (16MB).
Basically a cache has hits and misses. If everything goes well its a hit and if the cache misses data its a miss. If its a miss then the data needs to be fetched again. The cache is a very high speed form of memory which is much faster than normal RAM.
You have to find out how much missed data is involved for your 16MB file given your bit stream and your cache size (256KB). Have you been given formulae or assumptions for the hit and/or miss rate of each data packet?