Answer
10s
(10 seconds)
Work Step by Step
The signal is emitted when Clock A reads 10 seconds. We know that Clock A is located 4 clocks away from the origin. We were given that each clock is one light-second apart, so the signal from Clock A will take 4 seconds to reach the observer at the origin.
The observer's clock is 4 seconds behind Clock A based on the way we synchronized the clocks, so when the observer receives the signal his clock will read exactly 10 seconds.
