The amount that the collections worth increased by was $7.20. That tells you how much the additional dimes are worth that was added to the $9.45. So if there is $7.20 in dimes, there are 72 dimes that were added to the collection of dimes and nickles. The 72 dimes that were added was the doubling of the number of dimes that are apart of the collection. That means that if there was 72 dimes all together, there were 35 dimes in the initial collection, which are worth $3.50, , and 35 dimes were added later. If the total collection was worth $9.45, and $3.50 of that was in dines, then the difference of $5.95, which is the amount the nickles were worth. That means that there were 19 nickles that were apart of the initial collection.