A student used the mark-recapture method to assess the population size of grasshopper in a field. The student was asked to repeat the recapture procedure once on three consecutive days. The procedure followed by the student and the observations made are as follows:

A. On day one, 40 grasshoppers were captured, marked and released back in the field.

B. On day two, 60 grasshoppers were re-captured of which, 4 were marked. He marked the unmarked ones and released all 60 in the field.

C. On day three, 50 grasshoppers were re-captured of which 7 were marked. He marked the unmarked ones and released all 50 in the field.

D. On day four, 25 grasshoppers were re-captured of which 6 were marked.

The student was asked to calculate the population size based on the mean of the three observations. The estimated population size is: