On a long straight stretch of one-way single-lane highway, cars all travel at the same speed and all obey the safety rule: the distance from the back of the car ahead to the front of the car behind is exactly one car length for each 15 kilometers per hour of speed or fraction thereof (Thus the front of a car traveling 52 kilometers per hour will be four car lengths behind the back of the car in front of it.) A photoelectric eye by the side of the road counts the number of cars that pass in one hour. Assuming that each car is 4 meters long and that the cars can travel at any speed, let $ M$ be the maximum whole number of cars that can pass the photoelectric eye in one hour. Find the quotient when $ M$ is divided by 10.
1. **Define the problem and variables:**
   - Let \( s \) be the speed of the cars in kilometers per hour.
   - Each car is 4 meters long.
   - The distance from the back of one car to the front of the next car is \( \frac{s}{15} \) car lengths.
   - Therefore, the total distance occupied by one car and the gap to the next car is \( 4 + \frac{4s}{15} \) meters.

2. **Calculate the total distance traveled by the cars in one hour:**
   - In one hour, a car traveling at \( s \) kilometers per hour covers \( 1000s \) meters.

3. **Determine the number of cars that can pass the sensor in one hour:**
   - The total distance occupied by one car and the gap to the next car is \( 4 + \frac{4s}{15} \) meters.
   - The number of cars that can pass the sensor in one hour is given by:
     \[
     f(s) = \frac{1000s}{4 + \frac{4s}{15}}
     \]

4. **Simplify the expression:**
   - Simplify the denominator:
     \[
     4 + \frac{4s}{15} = 4 \left(1 + \frac{s}{15}\right)
     \]
   - Therefore, the function becomes:
     \[
     f(s) = \frac{1000s}{4 \left(1 + \frac{s}{15}\right)} = \frac{1000s}{4 + \frac{4s}{15}} = \frac{1000s}{4 \left(1 + \frac{s}{15}\right)} = \frac{1000s}{4 \left(\frac{15 + s}{15}\right)} = \frac{1000s \cdot 15}{4(15 + s)} = \frac{3750s}{s + 15}
     \]

5. **Find the maximum value of \( f(s) \):**
   - As \( s \) increases, the function \( f(s) \) approaches its asymptotic limit:
     \[
     \lim_{s \to \infty} \frac{3750s}{s + 15} = 3750
     \]
   - Therefore, the maximum number of cars that can pass the sensor in one hour is 3750.

6. **Find the quotient when \( M \) is divided by 10:**
   - The maximum number of cars \( M \) is 3750.
   - The quotient when 3750 is divided by 10 is:
     \[
     \frac{3750}{10} = 375
     \]

The final answer is \(\boxed{375}\)