One hat and Two shirts costs $21. Two hats and one shirt $18. Megan has exactly enough money to buy one hat and one shirt. How much money does Megan have?
Suppose Megan has enough money to buy 1 hat and 2 shirts for $21 and then another 2 hats and 1 shirt for $18. In total, she has bought 3 hats and 3 shirts for $39. But she has enough for only 1 hat and 1 shirt and so Megan has 39 ÷ 3 = $13.
