The problem states that there are 3 different types of cowpea seeds in the box, but it does not provide any information about the weight of each type of seed. Instead, the problem provides the weight of each individual seed for two of the three types. Therefore, we cannot accurately calculate the probability of selecting a seed that weighs exactly 0.8g from the box using the information provided.
If we assume that the problem meant to say that there are three seeds of each type, and that the weight of the third type is also 0.81g per seed, then we can calculate the probability of selecting a seed that weighs exactly 0.8g as follows:
Total weight of all seeds in the box = (3 x 0.81g) + (3 x 12g) + (3 x 0.8g) = 3.15g + 36g + 2.4g = 41.55g
Probability of selecting a seed that weighs exactly 0.8g = (3 x 0.8g) / 41.55g = 0.0727 or approximately 7.27%
However, since the problem does not provide clear and complete information, it is best to clarify the details or assume that something is missing before attempting to solve it.