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再次求助

Paul and Samuel are playing the same computer game on their own computers. At the start of the game, there are a certain number of targets on the screen. The targets will disappear from the screen one by one at a constant rate. Paul can shoot down the targets twice as fast as Samuel. Given that at the end of the game, Paul shot down 54 targets while Samuel shot down 36 targets. Find the number of targets at the start of the game..

108.
Clue: assuming the disappearing rate is one target/one second..