The mass of a seal is 20 kg, which is one-twentieth of the mass of an adult seal?

Since the seal weighs 20 kilograms. This is one part.

And an adult seal is 20 parts.

We get the proportions:

20 kilograms – 1 piece;

X kilogram – 20 pieces.

Let’s multiply the parts of the proportions.

We get:

X * 1 = 20 * 20;

X = 400 kilograms – the weight of an adult seal.

Answer: The weight of an adult seal is 400 kilograms.