John received $550 in simple annual interest at a rate of 2.5%

John received $550 in simple annual interest at a rate of 2.5% on his Certificate of Deposit.
How much did he invest in the CD?

You didn't say how long it was invested, although it seems to be 1 year since you called it annual interest. I'll give you a general answer just to be complete, since it's not much more complicated and it's good to know.

For t years, the interest factor would be just .025t, so divide that into the interest earned.

Principal = 550/.025t
For 1 year, this would be 22000

For 2 years, 11000 etc.