Hi, can someone please help me figure out how they got the answer:

The number of minutes between trucks arriving at a checking station are shown to follow an exponential distribution, with an average of fifty minutes between trucks arriving. The administrators of the station are concerned about congestion. It requests information on the probability that at least twenty minutes will occur between truck arrivals. You calculate this to be:

and the answer is .67

and i keep trying to get the answer but i can't here's my working:
and that's wrong, can someone please show me how to do this?