No worries at all, the mistake you made is extremely common. The best way to think about it is that the tariff is 1/4, and the existing value is 4/4 so that's 5/4 total which means that you can consider the tariff to be 1/4th the original cost or 1/5th of the new cost - both answers are correct if you're careful in your wording.
In my initial answer I also referenced fence posts. If you're thinking about building a fence with ten segments how many posts do you need? Well, to the right of each segment there's one fence post - but you've also got one that's not to the right of any segment so you can either describe your fence as ten segments long or eleven posts long.
This shit trips up people all the time so no worries.
I'm pretty sure it isn't true and assume Trump misunderstood a general explaining gleefully that since the US manufacturers repair parts any country at war with America would quickly find its fleet inoperable due to the immense constant maintenance required for these planes.
But, the President claiming there is a kill switch should cause all purchasing countries to cancel their orders - if such a switch exists then there's a backdoor somewhere in the software and the US sucks balls at espionage so China knows how to trigger it.