Let 's' represent the number of post cards that Sally had and 'm' the number of cards that Marta had.

Sally and Marta had the same number of post cards means s = m.

After Sally sold 18 of her post cards, Marta had 4 times as many post cards as Sally means:

m = 4(s - 18)

or we can write

m = 4(m - 18) (since s = m)

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click here to see the equation solved for m

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m = 24 cards

Sally and Marta had 24 cards each.