Atown had about 2120 acres of pine trees 40 years ago. only about 13% of the pine trees remain. how many acres of pine trees remain?

For a short time after a wave is created by wind , the height of the wave can be modeled using y= a sin 2pi(t) / t where a is the amplitude and t is the period of the wave in seconds. how many times over the first 5 seconds does the graph predict the wave to be 2 feet high?