Which of the tables represents a function?
table a
input
output
8
−9
8
−8
10
−8
table b
input
output
4
11
7
12
7
13
table c
input
output
−4
0
−3
3
−2
5
table d
input
output
10
0
10
4
3
7
table a
table b
table c
table d

The life, in years, of a certain type of electrical switch has an exponential distribution with an average life b = 2. if 100 of these switches are installed in dif- ferent systems, what is the probability that at most 30 fail during the first year?