Explanation:
a probability is always
desired cases / totally possible cases
in total we have 75 marbles, so, 75 different possible results when picking a marble.
25 of these 75 marbles are red.
50 of these 75 marbles are blue.
and can you already see, where we are going with this ?
a)
the probability to pick a red marble is
25/75 = 1/3 = 0.3333333...
b)
the probability to pick a blue marble is
50/75 = 2/3 = 0.6666666...