Lao Zhang has a house worth 400 thousand. Because of the urgent need for cash, I sold it to Lao Li at a 10% discount. After a while, the house price went up 10.

Because Lao Zhang sold it to Lao Li at a 10% discount, he lost 40x (1-90%) = 4 (ten thousand yuan).

Later, house prices rose by 10%. If Lao Zhang bought it, he lost 40x 10% = 4 (ten thousand yuan).

Then Lao Zhang lost 4+4=8 (ten thousand yuan)