The population of goldfish in a marine farm is decreasing at the rate of 20 percent per year. the farm started with a goldfish population of 1,000.

Ifound your complete question in other sources.  see attached image.  part a:   the equation modeling the problem is an exponential equation of the form:   y = a (b) * ^ t  where,  a: initial amount  b: decrease rate  t: time  substituting values we have:   y = 1000 (0.80) ^ t  part b:   for this case we substitute the value of y = 640 in the equation:   640 = 1000 (0.80) ^ t  we cleared t:   640/1000 = (0.80) ^ t  log0.80 ((0.80) ^ t) = log0.80 (640/1000)  t = log0.80 (640/1000)  t = 2 years