You have to convert the question, hint and answer environment.
What the block environment does is to save the content from the begin to the end block and hides it when you use \hideblocks[…]. To output the content of the environments you can use \useblocks[…].
\defineblock[question]
\defineblock[hint]
\defineblock[answer]
\defineenumeration[question][alternative=hanging,text={Question},stopper={.}]
\defineenumeration[hint] [alternative=hanging,text={Hint for question},stopper={.}]
\defineenumeration[answer] [alternative=hanging,text={Answer to question},stopper={.}]
\keepblocks[question]
\hideblocks[hint]
\hideblocks[answer]
\starttext
\section{Questions}
\beginquestion
\startquestion
Compute $4\times 5$.
\stopquestion
\beginhint
\starthint
Use the fact that $2\times 5 = 10$.
\stophint
\endhint
\beginanswer
\startanswer
$20$.
\stopanswer
\endanswer
\endquestion
\beginquestion
\startquestion
What is the value of $100 / 10$?
\stopquestion
\beginhint
\starthint
Remember that $10\times 10 = 100$.
\stophint
\endhint
\beginanswer
\startanswer
$10$
\stopanswer
\endanswer
\endquestion
\section{Hints}
\useblocks[hint]
\section{Answers}
\useblocks[answer]
\stoptext
Wolfgang