According to the above analysis of the experimental steps, the remaining steps can be supplemented as follows:

(3) Add pebbles to the right plate, add water to the beaker on the left plate, and add and subtract the water quality in the cup with a dropper until the balance is rebalanced;

(4) Pour the water in the left cup into the measuring cylinder and record the volume v1of the water in the measuring cylinder;

(5) pouring a proper amount of water into the measuring cylinder, recording the volume as v2, tying pebbles into the water in the measuring cylinder with a thin thread, and recording the total volume as v3 to obtain the volume of pebbles as V3-V2;

(6) The mass m of pebbles = the mass m' of water in the beaker on the left,

M = m' = water V 1,

The volume of small stones V=V3-V2,

Density expression of small stones: ρ=mV=ρ water V 1V3? V2。

It can be seen from the analysis that the equivalent replacement method is mainly adopted here, and the quality of water is used to replace the quality of small stones. In concrete experiments and life, such examples are: replacing the resultant force of two forces with one force, and replacing two resistors in series with one resistor.

So the answer is:

(3) adding water, and adding and subtracting the quality of the water with a dropper;

(4) Pour the water in the beaker into the measuring cylinder and measure the volume of V 1;

(5)V3-V2；

(6)ρ water V 1V3? V2。

For example, one force replaces the resultant force of two forces, and one resistor replaces two series resistors.