Each light bulb lasts an exponential amount of time, with parameter=1/2=1/2(in years). 5 light bulbs are installed in a room.

1). What is the probability that after 3 years there are at least 2 burned light bulbs?

2).What is the probability that after 3 years there is at least one working light in the room?

3). What is the expected number of lamps still working after 3 years?