Which answer best explains how to solve this problem?

during a swap meet, ethan sold 12 cards and bought 20 more cards. he had 50 trading cards when he left the swap meet.

how many cards did ethan have before the swap meet?

a.
work backward
ethan ended up with 50 trading cards.
before ethan bought 20 cards, he had 50 + 20 = 70 cards.
before he sold 12 cards, he had 70 – 12 = 58 cards.
ethan had 58 cards before the swap meet.

b.
work backward
ethan ended up with 50 trading cards.
before ethan bought 20 cards, he had 50 – 20 = 30 cards.
before he sold 12 cards, he had 30 + 12 = 42 cards.
ethan had 42 cards before the swap meet.

c.
work backward
ethan ended up with 50 trading cards.
before ethan bought 20 cards, he had 50 – 20 = 30 cards.
before he sold 12 cards, he had 30 + 12 = 48 cards.
ethan had 48 cards before the swap meet.