Exponential Growth & Decay.

4) The number of dandelions in a field grows based on the number present. On the 4th day of measuring, there were 140 dandelions. On the 15th day, there were 407 dandelions. a) Find an equation for the number of dandelions after t days. Find the starting value of dandelions (on day 0) when writing the equation. b) How many dandelions are in the field after 200 days? c) The dandelions can be controlled so long as the number stays below 20,000. On what day will that number be surpassed