I am modelling a flash memory system where 0.3 requests are writes (and so take 100 cycles to complete) and 0.7 are read-only (and so take 50).
The system can handle 4 requests at once (the proportions are based on observed values but the timings and the 4-at-once are arbitrary).
By my calculation, and assuming the system is "fully loaded" this means 0.2401 probability of 4 read-only, 0.4116 of 1 write and 3 read-only, 0.2646 of 2 writes and 2 read-only, 0.0756 of 3 writes and 1 read-only and 0.0081 of 4 writes.
That all, happily, adds to 1 - but is it correct? My combinatorics is very rusty and online calculators don't allow for proportions.