Each year for 4 years, a farmer increased the number of trees in a

Say the number of trees at the beginning of the 4 year period was x, then:
At the end of the 1st year the number of trees would be \(x+\frac{1}{4}x=\frac{5}{4}*x\);
At the end of the 2nd year the number of trees would be \((\frac{5}{4})^2*x\);
At the end of the 3rd year the number of trees would be \((\frac{5}{4})^3*x\);
At the end of the 4th year the number of trees would be \((\frac{5}{4})^4*x\);
At the end of the \(n_{th}\) year the number of trees would be \((\frac{5}{4})^n*x\);

So, we have that \((\frac{5}{4})^4*x=6,250\) --> \(\frac{5^4}{4^4}*x=5^4*10\) --> \(x=4^4*10=2,560\).

Answer: D.

If the question were "if all of the trees thrived and there were 6250 trees in the orchard at the end of 15 year period, how many trees were in the orchard at the beginning of the 4 year period", then we would have that: \((\frac{5}{4})^{15}*x=6,250\) --> \(x\neq{integer}\), so it would be a flawed question.

Hope it's clear.