A cell is placed in a medium containing radioactively labelled thymidine. After the cells undergo replication 3 times, what percentage of the cells will have both strands of DNA labelled?

Keep in mind that question asks about cells with label on both strands. Question is based on the Meselson & Stahl experiment. Due to semiconservative mode of replication, In each round of replication, one strand is old and one is synthesized new in newly replicated DNA (see fig below). Red strand represents the labeled strand.