In one day Jamaica can produce 12 coolers, or 6 radios, or some combination of the two. So you need to look at the tradeoff between coolers and radios.
If Jamaica was a one extreme producing 6 radios, it would have to give up all 6 to produce only coolers, where they can produce 12. This is the opportunity cost, how much of one good do you have to give up to produce exactly on unit of another. For Jamaica 6 radios per 12 coolers = .5 radio per 1 cooler.
For Norway 3 radios per 24 coolers = 1/8 radio per 1 cooler, or .125.