I need help with #16, #17 and #18

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

If f(x) = 3x2 - 2x+4 and g(x) = 5x + 6x - 8, find (f-g)(x).

Harold wrote this equation to model the level of water in a pool over time. the variable x represents time in hours. f(x) = 3,500 - 225x which statements about the situation are true? check all that apply. the water level is rising. the water level is falling. the initial level of water in the pool was 3,500 units. the initial level of water in the pool was 225 units. the pool was empty after 14 hours. the water was 2,600 units high after 4 hours.