Help with this math question for my fifth graders?

I have no idea how the book is saying the answer is 27 plants. I keep getting 31. here is the word problem.





Ms. Black has one begonia plant. If she buys two begonia plants each year for every one she has, how many will she have in four years?





Explain!!

Help with this math question for my fifth graders?
In the first year there's 1 plant.


In the 2nd year: 1 + 2 = 3 plants


In the 3rd year: 3*2 + 3 = 9 plants


In the fourth year : 9*2 + 9 = 27 plants





assuming none of them die of course - let's hope Ms. Black has Green fingers!
Reply:Year one; 1 + 2 = 3


Year two; 3 + 6 = 9


Year three; 9 + 18 = 27


Year four; 27 + 54 = 81





I get 27 in year three.





If you start with one in year one, it works out to 27 in year four.
Reply:Step by step:





1st year: 1 plant


2nd year: 1 + 2*1 = 1 + 2 = 3 plants


3rd year: 3 + 2*3 = 3 + 6 = 9 plants


4th year: 9 + 2*9 = 9 + 18 = 27 plants





However from the question it sounds like she has 1 plant at the start, and she'll be buying 4 times. In that case just do it one more time. You get the answer after just three steps though.
Reply:in year 1 she has one plant





in year 2 she buys 2x1=2 plants, added to the 1 she has makes 3





in year 3 she buys 2x3=6 plants added to the 3 she has makes 9





in year 4 she buys 2x9=18 plants added to the 9 she has makes 27
Reply:If she buys 2 plants for every plant she already has and keeps the originals, that means she is effectively tripling the amount of plants she has every year. Starting from 1, after 4 years it would be 1*(3^3)=27
Reply:1 for the first year, 2 for every one added to existing amount


1 + 2 = 3 second year she has 3


3 + 6 = 9 third year she has 9


9 + 18 = 27 fourth year she has 27