For every 15 minutes, there are 3 minutes of commercials We can write this as

Now in one hour, there are 60 minutes; therefore, in 1 hour and 20 minutes, there are 60+ 20 = 80 minutes.
Let us call x the number of minutes in these 80 minutes, then we have the equation

which says that the ratio of commercial minutes and the television minutes must remain constant.
Cross multiplication gives


dividing both sides by 15 gives

Hence, in 1 hour and 20 minutes, there are 16 minutes of commercial.