Question 614871
The house is worth $10,000.
Presumably this is what the house cost her.
She sells it for $11,000.
She buys it back for $9,900.
Her net profit after both transactions is equal to $11,000 - $9,900 = $1,100.
She started with a house.
She finished with a house.
She has an additional $1,100 on top of that.
$11,000 - $10,000 = $1,000 / $10,000 = her 10% profit.
$11,000 - $9,900 = $1,100 / $11,000 = his 10% loss.