The universe contains only about one-tenth as many helium atoms as hydrogen atoms. But a hydrogen atom is only one-quarter the mass of a helium atom. So approximately what is the ratio of the universe's helium to hydrogen by mass

the ratio of the universe's helium to hydrogen by mass is 2:5 approximately.

Explanation:

To solve this question, we assign arbitrary values to these elements in tandem to the data given in the question.

nH, Number of Hydrogen atoms = X = 100

nHe, Number of Helium atoms = 0.1X = Y = 10

From the data given, by mass:

if the mass of Hydrogen atom = 1g = 100g

therefore, the mass of a Helium atom = 4g (1*4) = 40g

Mass ratio of Helium to Hydrogen = 40:100 = 4:10 = 2:5 approximately