I'm not sure exactly which type you are meaning since there are four types of expansions, but depending on which specific type here are your answers! :)
I think there are 125 heart stickers and 375 star stickers. 20/4 = 5, so there are 5 heart stickers per sheet. there are 25 sheets in the package, 5 x 25 = 125. if the remaining stickers on a sheet are stars then there are 15 star stickers per sheet. 15 x 25 = 375.
To find the answer, you should divide 405 by 50 to find the mass of one coin. The formula should look like this:

= 8.1
The exact mass is 8.1 grams, but because you want an estimate, the answer should be
About 8 grams for the mass of 1 one-dollar coin