In the end of episode "Once in Love with Amy", Mr. Feeny gives the class a good math problem, despite being their social studies teacher: "One person washes a car in 6 minutes, the other in 8 minutes, how long does it take them to wash it together?"
The real answer to this problem is 3 minutes and 26 seconds.
To find out how much they can do together per hour, add together what they can do individually per hour: 1/6 + 1/8 = 7/24.
They can do 7/24 of the job per hour. Now I'll let t stand for how long they take to do the job together.
Then they can do 1/t per hour, so 7/24 = 1/t.
Flip the equation, and you get that t = 24/7 = 3.428 minutes or approximately 3 minutes and 26 seconds.
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Of interest, MentalFloss shares 25 Things You Might Not Know About 'Boy Meets World'.