This is a logical mistake: I borrowed 500 yuan from my parents, and then I paid it back 10 yuan. Actually, I borrowed 490 yuan, 490+490=980 yuan, and bought a pair of leather shoes with 970 yuan. I took 10 yuan, 970+ 10 = 980 yuan, which is exactly the same. You can't calculate by topic, that's a logical error, and it doesn't add up to practical significance.