This is an exponential density ....

, for x>0.

The mean is , which 20 for your problem.

you can compute that via parts...

.

For (b), you want the value that puts .5 probability to the right and left, let's call that m for median...

I do have a problem with (c) and (d). The number of people doing anything is a discrete rv and this is a continuous distibution. This does cause a problem with the wording. Between 10 and 20 isn't clear. I don't know if 10 and 20 are included. If so you just need to calculate...

and