Answer: The probability is P = 0.056
Step-by-step explanation:
The invited ones are:
Haroldo, Xerxes, Regina, Shaindel, Murray, Norah, Stav, Zeke, Cam, and Georgia
So we have a total of 10 persons.
The case where Xerxes arrives first and Regina arrives last is:
We have 10 slots, each slot corresponds to who arrived in which time.
For the first slot we have only one option, this is Xeres.
For the next slot we have 8 options (because Xeres already arrived, and Regina must arrive at last)
For the next slot we have 7 options.
for the next one we have 6 options, and so on.
So the combinations are:
C = 1*8*7*6*5*4*3*2*1*1 = 40320
Now, the total number of combinations is:
for the first slot we have 10 options, for the second we have 9 options and so on.
c = 10*9*8*7*6*5*4*3*2*1 = 725760
The probability is the combinations where Xerxes arrives first and Regina rrives last divided the total number of probabilities.
P = C/c = 0.056
, you have to factor it as 
, not 
, because that will leave an extra 6x because of 8x - 2x.