A population of variety of tiny bush in an experiment field increased by 10% in first year, increased by 8% in second year but decreased by 10% in third year. If present number of bushes in experiment field is 26730, n number of variety of bushes in beginning was:

Let the number of bushes originally be 100
Number of bushes after one year
100 ==10% (↑) ==> 110
After second year it becomes
110 ==8%(↑) ==> 118.8
After third year,
118.8 ==8%(↓)==> 109.3
Now, according to the question
109.3 = 26730
1 = $$\frac{{26730}}{{109.3}}$$
So, 100 = $$\frac{{26730}}{{109.3}} \times 100$$    = 25000
Thus, number of bushes originally was 25000

NOTE:
You can take number of bushes originally as x then solve for the x