Engineers want to design brakes that will reduce stopping distances by 10%. how would this change the equation relating speed s, friction f, and stopping distance d?
s=√30fd

For the answer to the question above,  we will use this formula to solve this problem
d = kv^2 

plugging in, 
4.2 = k*10^2 
k = 0.042 

d = 0.042v^2 

the revoised equation will be 
d = 0.9*0.042v^2 , i.e. 

d = 0.0378v^2 

I hope my answer helped you