There are 10 total marbles and three are pink. So you are to determine the probability of choosing a pink marble, placing it back in the bag and choosing another pink marble.
The probability of choosing a pink marble is # of pink marbles to # of all marbles which is 3/10. Since we place the marble back in the bag, the probability is the same for the second time.
To determine the probability of both occurrences, we need to multiply 3/10 x 3/10 which is 9/100. This can be written as 9:100 as a ratio.