Determine the pressure due to water at the bottom of a tank which is 25 m deep and 4/5 full of water (density of water = 1000 kgm-3, g = 10 ms-2)

The radioactive isotope of lead, pb-209, decays at a rate proportional to the amount present at time t and has a half-life of 3.3 hours. if 1 gram of this isotope is present initially, how long will it take for 80% of the lead to decay? (round your answer to two decimal places.)