Given:
Hiroaki says that a constant of proportionality must be a whole number and cannot be a fraction or a decimal.
To find:
The explanation, why Hiroaki is incorrect.
Solution:
If y is directly proportional to x, then



where, k is the constant of proportionality.
It means, the constant of proportionality is the ratio of two directly proportional variables.
So, constant of proportionality can be any real.
Thus, the statement that a constant of proportionality must be a whole number and cannot be a fraction or a decimal is incorrect.
Hence, Hiroaki is incorrect because constant of proportionality can be any real.