I am sorry that I couldn't finish the problem. The best method I can think of for this problem is listing the cases
Try to find all the cases of him buying a toy. The number of those cases will be the denominator of your answer
Then find all the cases of him getting change for the 10 dollar bill before him buying his favorite toy. The number of those cases will be the numerator of your answer.
This might be a little tedious! I hope this helps!
