Tom sold a watch to Dick at a profit of 20% and Dick sold it to Harry at a profit of 30%. If Harry paid $780 for the watch, calculate how much Tom paid for the watch.


The working solutions:

Dick paid $ (100/130 * $780) = $600 for the watch

Hence, Tom paid $ (100/120 * $600) = $500

I don't understand why the workings are done in that way...Could someone clear my doubt by explaining the above solutions? You could even explain and do the questions using your own method...Thanks..