A model estimates that at the end of each year from 2015 to 2020, the number of squirrels in a population was $150\%$ more than the number of squirrels in the population at the end of the previous year. The model estimates that at the end of 2016, there were 180 squirrels in the population. Which of the following equations represents this model, where $n$ is the estimated number of squirrels in the population $t$ years after the end of 2015 and $t\le 5$?