is there options for the rules?
if he is adding 2half dollars every day and he started with 10, the equation will look like
y=2x+10
y being the total amount of half dollars that Marlin has after x amount of days.
In order to figure out how many half dollars he has after 15 days, you just have to plug in 15 to the x.
y=2(15)+10 which is 40