Sam has 1584 marbles. he gives some of the marbles to 8 friends. each friend gets 15 marbles. how many marbles are left with sam

The distribution of the amount of money spent by students for textbooks in a semester is approximately normal in shape with a mean of $235 and a standard deviation of $20. according to the standard deviation rule, how much did almost all (99.7%) of the students spend on textbooks in a semester?