# Question #7917e

##### 1 Answer

Here's why the volume of the final drop is equal to the sum of the volumes of the initial drops.

#### Explanation:

So, you know that you have two water drops, each having a different radius. The first drop has a radius of **3 cm** and the second one has a radius of **4 cm**.

Now, you can relate *mass* and *volume* using density.

Since you're dealing with two *water* drops, it's clear that the density term will be equal for both drops.

This implies that you have *different masses* of water in the two drops.

Now, when the two drops come together, the mass of the resulting drop will be equal to

This means that, if you want to find the *volume* of the final drop, you can write

Since you can treat the drops as *spheres*, the two volumes will be equal to

The final volume will thus be

If you take

This is equivalent to

Therefore,