Open Question: Probabilities using counting techniques? please help?

a)Laura, Dave, Monique, Marcus and Sarah are going to a party. What is the probability that two of the girls will arrive first?
b) What is the probability that the friends will arrive in order of ascending age?
c) What assumptions must be made in parts a) and b)

how do i solve these, can someone explain me the steps? the answers are:
a) 3/10
b) 1/20
c) Assuming that the friends arrive individually

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