[Grammar] At the age/at the cost of/at a cost of/at a speed of/at the speed of/at a or the pric

[z7655431]

1.
He died at the age of 80.
- Can I say at AN age of...?

2.
She saved him from the fire but at the cost of her own life.
- Can I say at A cost of...?

3.
A new computer system has been installed at a cost of £8, 000.
–Can I say at THE cost of…?

4.
The truck was traveling at a speed of 50mph.
-Can I say at THE speed of...?

5.
There particles travel at the speed of light. -Can I say at A speed of...?

6.
I bought the bicycle at a/the price of NT$ 3,000.
-Between "a" and "the", which is correct?

 

[MikeNewYork]

1-5 no.

6. I would use "the".