To find out how many weeks Timmy needs to work to earn $80.99, you can divide the total amount he wants by his weekly earnings.
\[ \text{Number of weeks} = \frac{\text{Total amount}}{\text{Weekly earnings}} = \frac{80.99}{20} \]
Calculating that:
\[ \frac{80.99}{20} = 4.0495 \]
Since Timmy can't work a fraction of a week, he will need to round up to the next whole number. Therefore, he will need to work for 5 weeks to earn at least $80.99.