# Sixth question of the practice exam

I don't understand why if the frequency with which the family cars pass is equal to 3 then the mean is 1/3. Why is it that the sum of the passing cars is not simply the sum of the frequencies?

check out the video "exponential Distribution" in section 5 of the course, That helped me understand the expected value for exponential distribution is 1/landa.

so the sum of exponential distributions' expected values would be 1/3 + 1/8.

It looks there is a mistake in the last question. Your are given expected frequencies of two types of cars. 3 and 8 respectively You have been told it follows Poison distribution. 3 per time period and 8 per time period. The answer should be 11.

Here is my take on this.

"the sum of the number of family cars and the number of single-member cars" shall increase as the number of family cars and the number of single-member cars increase.

"1/3 + 1/8" does not behave so therefore this cannot be the right answer