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
10/15 chance needed to do 2/3 puzzles
that's all i can work out sorry hope i've helped