Let's assume the initial number of orange trees on MacDonald's farm is \( t \).
According to the problem, MacDonald removed 5 orange trees, so the number of trees remaining after the removal is \( t - 5 \).
Each of the remaining trees produced 210 oranges, so the total number of oranges from the remaining trees is \( 210(t-5) \).
The problem also states that the total harvest was 41790 oranges. So, we can write the equation:
\( 210(t-5) = 41790 \)
To find the initial number of orange trees, we can solve this equation for \( t \):
\( 210t - 1050 = 41790 \)
\( 210t = 42840 \)
\( t = \frac{42840}{210} \)
\( t \approx 204 \)
So, the initial number of orange trees on MacDonald's farm was approximately 204.