The coefficient of static friction between hard rubber and normal street pavement is about 0.90. on how steep a hill (maximum angle) can you leave a car parked?

Ok, first, if the mass of the carr is M, and the angle of the hill is a (counting counterclockwise from the ground), the equations for the forces are:

Gravity force Fg = - Mg*sin(a)

Knowing that the normal force that the ground is doing in the car is N = Mg*cos(a)

then the Friction force is: Ff  = 0.9*Mg*cos(a)

if the car is parked, then the sum of the forces must be equal to zero, this is

0.9*Mg*cos(a) - Mg*sin(a) = 0

Mg( 0.9*cos(a) - sin(a)) = 0

so

0.9*cos(a) = sin(a)

0.9 = sin(a)/cos(a) = Tg(a)

a = aTg(0.9) = 41.9°

so the maximum angle is 41.9°, if is smaller or equal, then your car will be fine.

double the distance = divide strength by 4

the components of a vector are the parts of the vectors that are to each other.

b perpendicular