An engineer has observed that the gap times between...

#1
I am struggling with this question ..please help

An engineer has observed that the gap times between vehicles
passing a certain point on a highway have a exponential distribution with mean of 10 seconds.

1) Find the probability that the next gap observed will be no longer than 1 minute.

2)Find the probability density function for the sum of the next four gap times to be observed. What assumption must be true for this answer to be correct?