Final answer:
The constant of proportionality in the equation b=5/8 is 5/8.
Step-by-step explanation:
In a proportional relationship, the constant of proportionality represents the ratio between the two variables.
In this case, every input value of b is associated with an output value that is 5/8 of the input value.
Therefore, the constant of proportionality in the equation b=5/8 is 5/8.