Dylan has 765 cards in his baseball card collection. he sells 153 of the cards. what is the percent of decrease in the number of cards in the collection?

she sells means it is decrease in her collection, so out of 765 decrease is 153
then out of 100 decrease is 153/765 × 100 = 20 % 

In an elementary expalination, the total number of cards is

765 as an easy step 10% of the cards is 76. if you times 76 x 2 = 152, you will have 20%. Reasoning is 76 is 10% 2 x 76 is basically saying 10% twice of the total doubles to 20%. You can add the 10% 2 times and the closer you get to the amount sold, is the %. For example, if Dylan sold 304 cards, just say 76 x 4 =  304. So, the amount of time you timed it with the 10%, is the %. In this example it is 40%.