Learn Russian: Плотность этого металла — семь целых восемь десятых. - The density of this metal is seven and eight tenths.

These words are technically adjectives acting as nouns, standing in for целая часть (whole part) and десятая часть (tenth part). Since they follow the numbers семь (7) and восемь (8), Russian grammar requires them to be in the genitive plural form.

If the decimal was 7.2 instead of 7.8, how would this sentence change?

It would become семь целых две десятых. Because the hidden noun часть (part) is feminine, you must use the feminine form две instead of the masculine два. The same rule applies if a number ends in 1, which becomes the feminine одна.

Why is there a dash between металла and семь?

The dash replaces the present-tense verb 'is'. In Russian, when you link a subject (like плотность) directly to a definition, a noun, or a specific numerical value, a dash is used in writing much like an equals sign.

Do Russians always use this long format for decimals?

In formal contexts and science, yes. But in everyday speech, just as English speakers say 'seven point eight', Russians often say семь запятая восемь (seven comma eight, since Russian uses commas instead of decimal points) or simply семь и восемь.