I think it’s easier to see if you drop the time indices and just introduce a variable for the elapsed time. So:
Mary := 24 = 2(A - Δ)
A = Mary - Δ
And then it’s just substitution. But of course what appeals to one’s intuition is very personal
Mary := 24 = 2(A - Δ)
A = Mary - Δ
And then it’s just substitution. But of course what appeals to one’s intuition is very personal
M = 2A' "Mary is [now] twice as old as Ann was ..."
M'= A "when Mary was as old as Ann is now"