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 $t\le 5$?