Final answer:
We cannot determine the value of 'b' without additional information or context because the question provided does not specify how 'a', 'b', and 'p' are related.
Step-by-step explanation:
To find the value of b given that a = 3 and p = 20, we first need to understand the relationship between these variables. Unfortunately, the question does not provide a specific formula or equation involving these variables. Therefore, we cannot solve for b without this critical piece of information. Knowing that p equals the number of A alleles over the total number of alleles, which equals 0.7, and q equals the number of a alleles over the total number of alleles, which equals 0.3, indicates that p + q = 1. However, without an explicit connection to the variable b, we cannot determine its value.
It's possible that the question intends to reference Hardy-Weinberg equilibrium principles or another genetic-related equation, but without further context, we must acknowledge that we are missing information. Hence, it's best to ask the student for additional details or check if there has been an oversight in the posed problem. Once the proper equation or relationship is established, calculating the value of b is certainly achievable.