Calculate Percentage: Finding Remaining Fields from 17 to Water After Harvest

First, we'll find out how many of the fields the farmer still needs to water. The total number of fields the farmer has is 17, and 5 fields are ready for harvest and don't need watering. Therefore, the number of fields that still need to be watered is:

$17 - 5 = 12$

Now we calculate what percentage 12 fields represent of the total 17 fields. Using the percentage formula:

$\text{Percentage} = \left( \frac{\text{Number of fields that need watering}}{\text{Total number of fields}} \right) \times 100\%$

Substitute the values into the formula:

$\text{Percentage} = \left( \frac{12}{17} \right) \times 100\%$

Calculating this gives:

$\text{Percentage} = \left( 0.70588 \right) \times 100\% \approx 70\%$

Thus, the farmer needs to continue watering approximately 70% of the fields.

The correct answer, therefore, is choice 3: 70%.