Help with this math question for my fifth graders?

Question

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!!

Answer 1

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!

Answer 2

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.

Answer 3

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

Answer 4

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

Answer 5

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.

Answer 6

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