As a preparation I took the "Mendix Introduction Course" which takes about 30 minutes according to Mendix. This combines theory with Knowledge checks and it prepares you in a good way for the rapid developer exam since allmost all the answers to the questions asked in the Rapid Exam can be found here.
Oh and remember, there is no time limit on the Rapid Developer Exam and you can start, for example, tonight and continue tomorrow.
So take your time :) Good luck!
To start: having passed them with 80% is a good score!
About finding the right answers, atm it isn't possible to show them. Though use the question as a start point to go through the theory (video or text below the video) to have another moment spent on learning.
Secondly, the knowledge checks are a good representation of the exam. so, passing the knowledge checks is a good preparation.
The right answers are not available on the gettingstarted page, unless you have given them ofcourse.
You might have more luck by asking the correct answers. Send a email to: Academy@mendix.com
Edit: I recommend you to check https://docs.mendix.com
Second question: you'll get 80 questions all look-a-like the gettingstarted training.
Edit: 50 questions ;)
However they make sure that always 2 answers are (almost) correct and you'll need to choose the most correct one.
Hope this works for you.