To me, explicitly representing the two pairs of ages (now: M, A; previously: M', A') is simpler because it let's you directly represent the problem statement wi

To me, explicitly representing the two pairs of ages (now: M, A; previously: M', A') is simpler because it let's you directly represent the problem statement without any mental gymnastics whatsoever. M = 2A' "Mary is [now] twice as old as Ann was ..." M'= A "when Mary was as old as Ann is now"