OK, so let's figure it out then. We'll take the number of times that a plane has actually blown up due to someone detonating a bomb on it, and since we're talking about US travel, we need to limit it to instances that it has occurred in the US. Then we'll divide that by the number of commerical passenger flights over a given timeframe. I stated decades but I'll even give you a smaller timeframe if you'd like.
0/whatever number you want = 0.
To add a bit more detail, per the NATCA, there are roughly 29,000 commercial flights per day. This gives us 10.5 MILLION flights per year. Let's say we go back to the Lockerbie bombing (US destination, originated in London) in 1988. I'll guess a lower number of total flights due to there being less in prior years. That gives us 1/150,000,000 = 0.000000667% of flights that have been brought down by a bomb. Is that not basically zero?
The risk of anyone even attempting to bomb the plane you are on is ridiculously small. So small that it shouldn't even be considered by the person taking the flight. Emotionally we all fear these high profile events (shark attacks, plane crashes, etc) when looked at rationally the risk is neglible to miniscule.