You can only use the annuity equations when these conditions are met:

1. A series of uneven cash flows exists, spaced equidistantly in time.

2. A series of equal cash flows exists, spaced equidistantly in time.

3. A single cash flow exists, spaced at some place in time.

4. A series of three equal cash flows exists, one in 1 year, one in 3 years, another in 10 years.

(Could you also give a brief explanation of why the answer is what it is?)