I dont get this can you help me?

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?

The? half-life of a radioactive substance is 20 years. if you start with some amount of this? substance, what fraction will remain in 120 ? years

The angle of elevation from point a to the top of a cliff is 38 degrees . if point a is 80 feet from the base of the cliff , how high is the cliff ?