"When Mary was as old as Anna is now, Mary is twice as old as Anna was" - a rephrase of the second sentence.

Let x be the difference between the present ages of Mary and Anna. Then,
24 - A = x >>>> 24 - x = A eqn 1

Equation 2 comes from the second sentence of the problem.
" When Mary is as old as Anna is now" means when the age of Mary is 24 - x
"Mary is twice as old as Anna was" means 24 - x = 2 (A - x) eqn 2

Solving the two equations gives A = 16 years old

Yun nga lang, sa lahat po ng librong nakitaan ko ng problem na ito, ang sagot nga po talaga is 18 years old. Probably may mali sa pagkakaintindi ko ng second sentence :( . Di ko po talaga maintindihan.

I'm pretty sure the answer is 16. Shouldn't it be?

You may show your solution for us to evaluate.

Let A be the age of Anna in the present.

"When Mary was as old as Anna is now, Mary is twice as old as Anna was" - a rephrase of the second sentence.

Let x be the difference between the present ages of Mary and Anna. Then,

24 - A = x >>>> 24 - x = A eqn 1

Equation 2 comes from the second sentence of the problem.

" When Mary is as old as Anna is now" means when the age of Mary is 24 - x

"Mary is twice as old as Anna was" means 24 - x = 2 (A - x) eqn 2

Solving the two equations gives A = 16 years old

Yun nga lang, sa lahat po ng librong nakitaan ko ng problem na ito, ang sagot nga po talaga is 18 years old. Probably may mali sa pagkakaintindi ko ng second sentence :( . Di ko po talaga maintindihan.

Meron din itong sagot sir na 12. Hehehe.