Read specific data from a text file and calculate a result

Question

I have to read data from a text file like this:

time power status
0 10 on
10 9.9 on
11 9.7 on
12 9 on
13 6 on
14 4 on
15 -1 off
20 10 on
25 9 on
26 5 on
27 4