what's the constant of proportionality

2 5.00
3 7.50
4 10.00
8 20.00

The constant of proportionality is 2.50. This can be calculated by dividing the second column values by the first column values:

5.00 / 2 = 2.50
7.50 / 3 = 2.50
10.00 / 4 = 2.50
20.00 / 8 = 2.50