(2) Because mass and volume are two related quantities, one quantity changes and the other changes.

Their mass/volume =7.8.

So the two related quantities, mass and volume, are directly proportional.

2: ① 2×72= 144

② The number of sluice openings and the required flood discharge time are two related quantities, one of which changes and the other changes.

Their number of open gates × required flood discharge time = 144.

Therefore, the number of sluice gates and the required flood discharge time are two inversely proportional related quantities.

3.① Because quantity and total price are two related quantities, one quantity changes and the other quantity changes.

Their total price/quantity = 16.

So the quantity is directly proportional to the total price.

② Omission