We know that for each 13$ Rob earns babysitting, he saves 9$. Thus, we can separate the money he earns into groups of 13, and for each group of 13, we can add 9$ to his savings.
Assume that Rob has 78$ lying on a table. He grabs 13 of them, leaving 65, and puts 9$ to the side for savings. He does this 5 more times (as 65 divided by 13 is 5, so Rob would make 5 groups of 13 out of his 65$ that is left), meaning that he now has 5+1 (the original group) = 6 groups of 9. 9*6=54, so he saves 54$. Please let me know if you have any further questions!