1. I was at the beach at sunset.
2. She got married at 25.
3. She bought the backpack at 20 dollars.
4. She bought the backpack for 20 dollars.
5. He was at/on the corner of the street.
6. He was driving at 70 mph.
(Are they all grammatical? Can we use 'for' instead of 'at' as in #4? How do you read 'mph'?)
4 is much better than 3.
5 is correct with either preposition.
"mph" sounds just like that: "em pee ach" but "miles per hour" is also commonly used -- or simply "He was going 70 on that long lonely highway."