One of the problems of in-air testing of nuclear weapons (or, even worse, the use of such weapons!) is the danger of radioactive fallout. One of the most problematic nuclides in such fallout is strontium -90 (90^Sr), which breaks down by B- decay with a half-life of 28 years. It is chemically similar to calcium and therefore can be incorporated into bones and teeth, where, due to its rather long half-life, it remains for years as an internal source of radiation.
Part A What percent of the original level of is left after 56 years?
Part B How long would you have to wait for the original level to be reduced to 0.781% of its original value?
t=_____________ years