Упр.29 Unit 2 ГДЗ Enjoy English 10 класс

Приведем выдержку из задания из учебника Биболетова, Бабушис 10 класс, Дрофа: 29 Read the joke. Explain why was able to is used in it. Can it be replaced with could?
Jones: Aren’t your daughter’s piano lessons costing a fearful lot?
Brown: On the contrary, I was able to buy the house next door at half its worth.
*Цитирирование части задания со ссылкой на учебник производится исключительно в учебных целях для лучшего понимания разбора решения задания.

Ответ

was able to is used because it means that Brown managed to do something successfully on one particular occasion in the past. He bought the house next door at half its worth.

It cannot be replaced with could here, because could shows general ability in the past, not a single successful action.