To find out how many weeks Timmy will need to save up $57.85 if he earns $20 each week, you can divide the total amount he needs by the amount he earns each week.
\[ \text{Number of weeks} = \frac{\text{Total amount needed}}{\text{Amount earned per week}} = \frac{57.85}{20} \]
Calculating this gives:
\[ \frac{57.85}{20} = 2.8925 \]
Since Timmy can't work for a fraction of a week, you would round up to the next whole number. Therefore, he will need 3 weeks to earn at least $57.85.
So, Timmy will need 3 weeks to get $57.85.