It’s better to label these topics according to the general topic that we are discussing, rather than a question number, and to deal with them one at a time.
So, this one is about the topic of equivalent resistance in parallel circuits. (Please start separate topics about the other questions.)
We cover this in Unit 5 of Basic Electricity in the 3rd video and in the section below the video. You also were asked for the rule of thumb in the quiz for that unit.
Do you know the rule of thumb?
You set up the equation correctly, but did not calculate it right. 1/10 + 1/20 does not equal 2/30. I think this is easiest if you use your calculator to change the fractions into decimals. If you don’t know how to do all of that, you can just give the rule of thumb for the answer.