M3.4.8 For the adult
Inverse proportion
3 people dig a ditch in 8 days. How long would 6 people take?
What this chapter teaches
In inverse proportion one amount goes down as the other goes up, and the two multiply to the same number. Measure a job in person-days, then divide by the new number of people. This works only if everyone works at the same speed and the job can be shared out.
Three questions to ask
There is no one right wording. Listen for the idea.
- Why do 6 people take fewer days than 3 people to do the same job?
- How do you find the size of a job in person-days?
- How is this different from the rice and water that grew together?
After the chapter, the learner can
- Spot when one amount goes down as the other goes up
- Find how long 6 people take if 3 people take 8 days
What they copy into their notebook
In inverse proportion one amount goes down as the other goes up. Their product stays the same. Find the job in person-days, then divide by the new number of people. Everyone must work at the same speed.