Inverse proportion is to do with the rate at which one quantity increases as the other decreases.
The key to answering these problems is to consider 1 unit.
2 builders take 6 days to build a wall. How long would it take 5 builders to build a similar wall?
We need to consider 1 unit (i.e. 1 builder).
It would take 1 builder 2 times as long as it would take 2 builders.
So it would take 1 builder 12 days to build the same wall.
As the number of builders increases, so the time taken to complete the job decreases in inverse proportion.
So 5 builders take one fifth of the time it takes 1 builder.
It takes 5 builders 12 ÷ 5 = 2.4 days.
10 students take 10 days to pick the grapes in a vineyard
How many students would be needed to do the job in 20 days?