Answer:
The number of trees at the begging of the 4-year period was 2560.
Step-by-step explanation:
Let’s say that x is number of trees at the begging of the first year, we know that for four years the number of trees were incised by 1/4 of the number of trees of the preceding year, so at the end of the first year the number of trees was , and for the next three years we have that
, and for the next three years we have that
                              Start                                           End
Second year      --------------
 --------------   
Third year     -------------
-------------
Fourth year   --------------
--------------
So  the formula to calculate the number of trees in the fourth year  is  
 we know that all of the trees thrived and there were 6250 at the end of 4 year period, then
 we know that all of the trees thrived and there were 6250 at the end of 4 year period, then  
 ⇒
⇒
Therefore the number of trees at the begging of the 4-year period was 2560.