Learn Russian: Этот завтрак с пятьюстами калориями кажется полезным. - This breakfast with five hundred calories seems healthy.

The number пятьсот (500) is made of two parts: пять (five) and сот (hundreds). When declining hundreds in Russian, both halves change. The instrumental of пять is пятью, and the instrumental ending for the hundreds part is -стами. Combined, they make пятьюстами.

Why is калориями in the instrumental case instead of the genitive plural?

When a number is in the nominative or accusative case, the noun following it goes into the genitive plural (like пятьсот калорий). However, when the number is in any other case, the noun simply matches the case of the number. Because the preposition с (with) requires the instrumental case, both the number and the noun take the instrumental: с пятьюстами калориями.

Why is the adjective полезным in the instrumental case if it describes the nominative subject завтрак?

The verb казаться (to seem) requires the adjective describing the subject to be in the instrumental case. Even though завтрак is in the nominative, the trait it seems to possess takes the instrumental: кажется полезным. You will also see this pattern with verbs like быть (to be) and стать (to become).

Is there a way to say this without declining the number 500?