答案：B

解析：

From the information given, you can write the proportion 3 cars is to 4 hours as 5 cars is to x hours, where x represents the number of hours the mechanic will take to install carburetors in 5 cars. This gives 3 cars over 4 hours = 5 cars over x hours, which simplifies to 3 times x = 4 times 5 = 20. Therefore, x = 20 over 3 = 6 and (2 over 3) hours. Since 2 over 3 hour is 40 minutes, x is 6 hours 40 minutes.