Mom and dad borrowed 500 each, that is, borrowed 1000, but paid back 20, actually owing 980, so the mentality that the final sum is equal to 1000 is wrong and not lacking.

1000 (borrowed money) =500 (dad) +500 (mom) =970 (shoes)+10 (repayment of dad)+10 (repayment of mom)+10 (my surplus).

=490 (owed to dad) +490 (owed to mom)+10 (returned to dad)+10 (returned to mom)

In this way, it is clear at a glance that the misunderstanding of the topic is to put two unrelated equation numbers together and give you a misunderstanding of thinking.