It takes Russel 20 minutes to inspect a car and Sohel needs 18 minutes to inspect a car. They both start inspecting cars separately at 10:00 am. At certain points of time, both of them will finish inspecting a car at the same time. When will this occur for the first time?

We need to find the LCM of 18 and 20 in order to get the time interval when they both end inspection simultaneously. LCM of 18 and 20 is 180. That means after 180 min, they both end their respective inspection simultaneously. As they started at 10 AM, so after 180 min 0r 3 hr, the time will be 1:00 PM.