Let's find the rate of change of each case.
We have to use the following formula

For function a, let's replace the points (0,0) and (5, 50), where


The rate of change for function a is 10.
Let's find the rate of change for function b using the points (3,33) and (5,55) where


As you can observe, function b has a greater rate of change by 1 unit.