Ok

Im no genius but here it goes

So bassicly he can complete 3 out of every 5 puzzles he does.

There are 4 weeks we need to work out if he can do 2 puzzles in 3 weeks so the 4 weeks isn't relevant?

now im gonna find the lowest common denominator

so this means theres a

9/15 chance he can do a puzzle

and a

10/15 chance needed to do 2/3 puzzles

that's all i can work out sorry hope i've helped