1 1/6
3/5
You find a common denominator:
It is 30.
So, 5/30 and 18/30.
5/30 + 18/30 = 23/30.
Therefore, the third commercial will be 7/30 of a minute long, because 30/30, or 1 whole, - 23/30 = 7/30
Therefore, your answer is 7/30.
Glad I could help, and good luck!