NO_x, a mixture of NO and NO₂, plays an essential role in the production of pollutants found in photochemical smog. The NO_x in the atmosphere is slowly broken down to N₂ and O₂ in a first-order reaction. The average half-life of NO_x in the smokestack emissions in a large city during daylight is 3.9 hours.
(a) Starting with 1.50 mg in an experiment, what quantity of NO_x remains after 5.25 hours?
(b) How many hours of daylight must have elapsed to decrease 1.50 mg of NO_x to 2.50 × 10⁻⁶ mg?