Get 500+500= 1000. Expenditure 970+20=990. I have 10 left, which is exactly 1000.

That 30 is the rest of the shoe. If you pay them back 20, they still owe 980. You buy shoes for 970 plus 10. Isn't it just right to hold them in your hand? Where is 10? 20 for parents is the rest, not flowers.