Question

Bobby has a party while his parents are away for the weekend. Bobby estimated that if his 9 friends hep him to clean the house, they will finish in time if they start 8 hours before his parents return. But after only two hours of cleaning, Bobby's parents call and say they are just two hours from home. How many more friends does Bobby need to call to help finish cleaning in time.

Answer 1

Using an inverse rule of three, it is found that Bobby needs to call 18 more people.

What will be the number of friends?

This question is solved by proportions, applying a rule of three.The number of people working and the time needed to complete the work are inversely proportional, as more people means less time, thus, instead of cross multiplication, we multiply the lines, which is an inverse rule of three.When Bobby's parents call, with 9 people, they will need 6 hours to finish the job. How many people are needed to finish the job in 2 hours?

The rule of three is:

9 people - 6 hours

x people - 2 hours.

Multiplying by line:

2x  =  6  x  9

x   =   3  x   9

x    =  27

Already has 9 people, in total needs 97, so he needs to call 27 - 9 = 18 more people.

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