Answer:
 constant of proportionality = 2.5 
Step-by-step explanation:
Given  :  y = 2.5x.
To find : What is constant of Proportionality in the equation.
Solution : We have given 
y = 2.5x.
On comparing with equation of proportionality : y = kx, 
Where,   k is the constant of proportionality.
On comparing y = 2.5 x  by y = kx .
We get  k = 2.5 
So, constant of proportionality = 2.5 
Therefore, constant of proportionality = 2.5