Page’s thought process:
From Peter’s statements in the third panel, exchanging 1 shirt for 1 sweater costs an extra $15 therefor exchanging the second shirt for a third sweater will make the bill $90.
Sweaters obviously cost $30 apiece.
Going back to the first statement and subtract 1 sweater and 2 shirts cost $30 or $15 apiece.
Last night upon the stair I saw a little man who wasn’t there He wasn’t there again today Oh, I wish he’d go away.