$58 bill; 20% tip. how do i do this?

I hope you mean "How do I find the total amount I need to pay?" If so, please resume.

First, you need to find what the tip is by converting 20% to its decimal form, 0.2 and multiplying that by 58 to get 11.6. 

Then, add that to 58 to get the total amount, tip included. The final answer is $69.40.