According to this question it is assumed that the total calls are 420. it says 400 calls connected, while 20 were blocked, meaning those 20 never made it through.
so we have a total of 420 offered calls
400 successful calls
and 20 blocked calls
The formula for finding the Grade of service (GoS) can be written as
Number of blocked calls / Number of offered calls.
20/420 = 0.477