Answer 1

Master One
If you are really playing Challenges for knowledge then atleast you should complete all modules for basics...

Although if you are playing for fun they no need to complete any lesson.. even not the first lesson... Coz you will loose all Challenges in starting (around 1 month) after that you will surely score 5 in each challange..

My recommendation will be to complete lessons..

Because Challenges are basically designed for "giving a basic knowledge"