Crew A can build a complete car in 20 minutes and Crew B can build

New Correct solution:

Well this one got me too, and this is why I am keeping the original answer posted. I also missed the part of A and B working independently not together. But, since they are not working together, my above solution is wrong.

Solution to the problem when both A and B are working independently is much easier, with almost no calculation required.
It's given that A can build a car in 20 mins, and B can build a car in 25 mins. So, Work rate of A is > that of B. So, if they work for similar no. of hours, A would make more than B does. Since, the #no. of cars is going to be an "integer", we have to take the no. of minutes as the LCM of the (20, 25). That is 100 mins. So, for 100 mins, A would produce 5 cars (\(\frac{100}{20}=5\)), and B would produce 4 cars (\(\frac{100}{25}=4\)). So, that is 9 cars. Now the question asks us the time required to make 10 cars. Since, we have 9 out of 10 cars already, one car left would be made by A in lesser time than B. So, in another 20 minutes we will have one more car made by A. So, the total time taken to get 10 cars when both A and B are working independently is 100 + 20 mins = 120 mins. \(Option (D).\)