Answer
$r=40cm$
Work Step by Step
Using the relation that $$r=2f$$ and substituting the value $f=+20cm$ yields $$r=2(20cm)=40cm$$
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.