Answer:
The value is

Step-by-step explanation:
From the question we are told that
The gradient is G = 2% = 0.02
The initial speed of the vehicle is u = 40 mph
Generally the breaking speed is mathematically represented as
![d = (u^2)/(30 [(a)/(g) - G ])](https://img.qammunity.org/2021/formulas/physics/college/f6ha1w1kh0rs9pqrcvadfcl0qzt7a5s4bh.png)
Here
is equal to longitudinal friction coefficient which has a value of

So
![d = (40^2)/(30 [0.35 - 0.02 ])](https://img.qammunity.org/2021/formulas/physics/college/aw522m7q4e5a9dw69sh2dlzuy0q2wxie6u.png)
=>
