Answer:
Probably about 56 g/mol.
Step-by-step explanation:
Rearden is a fictitious metal invented by Hank Rearden in Ayn Rand's 1957 novel, Atlas Shrugged.
It is a <em>substitutional alloy of iron and coppe</em>r, with some <em>interstitial carbon </em>to harden it and perhaps some <em>titanium</em> for lightness and strength.
It must have a strange, open crystal structure to give it a low density.
Most steels are 96 % iron (A_r ≈ 56).
Copper (A_r ≈ 64) and titanium (A_r ≈ 48) would be minor components, even if they constitute more than 4 % of the mixture.
The average relative atomic mass is probably close to 56.