Then, for the guest, Xiao Zhang gave the guest a pair of shoes worth 20 yuan, and gave him real money, 20 yuan!
Then for the neighbor, Xiao Zhang had no money, and now he has 30 yuan to sell shoes (the neighbor gave him 50 and gave him 20), but he has to give his neighbor 50, so he has to pay 20 out of his own money!
To sum up, Xiao Zhang finally gave a pair of shoes to 20 yuan and his money to his neighbor 20 yuan!
20 plus 20 equals 40, and he lost 40 dollars!
Or it's easier to understand this way. First of all, if the neighbor didn't want to compensate, Xiao Zhang earned 10 yuan, but now the neighbor wants Xiao Zhang to compensate 50 yuan, so Xiao Zhang should be+10+(-50)=-40, which means Xiao Zhang should compensate 40 yuan!