It takes 7 people 8 hours to build 3 cars. How many hours will it take 2 people to build 1 car? Thank you!
It takes 7 people 8 hours to build 3 cars.
It takes 1 person 7 times as long to build 3 cars as it does 7 people.
Therefore it takes 1 person 7×8 or 56 hours to build 3 cars.
It takes 1 person only 
as long to build just 1 car as it takes
for him to build 3 cars.
Therefore it takes 1 person ×56 or 
hours to build 1 car.
It takes 2 people only half as long to build 1 car as it takes 1 person.
Therefore it takes 2 people ×
or 
or 
hours to build 1 car.
Or you can do it by this formula:
where
W1 = the number of workers (or machines) in the 1st situation.
W2 = the number of workers (or machines) in the 2nd situation.
T1 = the number of time units (hours, days, etc) in the 1st situation.
T2 = the number of time units (hours, days, etc) in the 2nd situation.
J1 = the number of jobs done in the 1st situation.
J2 = the number of jobs done in the 2nd situation.
In your case
W1 = 7, T1 = 8, J1 = 3
W2 = 2, T2 = ???, J2 = 1
Where T2 is the unknown, so we can substitute in the formula:
Cross multiply:
or 9 hours and 20 minutes.
Edwin
