IMAS 2023–2024 — Middle Primary — Round 2 Question 11

There is a row of lights. Every 10 seconds one of the lights, the “active” light, is switched on if it is off, and off if it is on. Then the next light in line becomes the active light. When the last light was active, the next active light was the first light. For example, the row of three lights, with the first and third lights on and the second one off, and the first one active, then after 10 seconds the first one would be switched off and after another 10 seconds the second one would be switched on, and so on. The sequence is illustrated below, where white circle indicates on and black circle indicates off.



Now, there is a row of 10 lights. At start, the first light was active and the switches were



After 2024 seconds, how many lights were on?