Two Cu+ ions can be replaced by one Cu2+.
The compound should have been Cu2S as ideally all the copper ions should have been Cu+.
Cu1.8S - for every mole of S2−, x moles are Cu2+ and the rest i.e. (1.8-x) moles of Cu+ are present.
Using the idea of charge conservation:
Net charge on cations =Net charge on anions
⇒Charge on ( Cu++Cu2+ )= Charge on S2−⇒x×2+(1.8−x)×1=2⇒x+1.8=2⇒x=0.2
0.2 moles of Cu2+ have replaced 0.4 moles of Cu+ in 1 mole of compound.
∴ % of Cu2+ in the total copper content