Percentage change between two numbers A & B can be
calculated as:

(B-A)/A * 100

For example, if a stock price increases in value from $123 to
$145 the percentage increase is:

(145-123)/123 * 100 = 17.9%

Alternatively, if a stock price decreases in value from $145 to
$123 the percentage decrease is:

(123-145)/145 * 100 = 15.2%

The technically correct answer to the scenario of $145 stock
dropping to $123 is a -15.2% change in price. The original answer
is semantically correct because the user calls out percentage
“decrease”, but you cannot have a drop in value and a positive %
change outcome.