I can't test this but I think it would be
Dim Myarray(50,50)
index=10
value=Myarray( (index – (index mod 50))/50 , index mod 50 )
print "value at index=10 is ";value
Dim Myarray(50,50)
index=10
value=Myarray( (index – (index mod 50))/50 , index mod 50 )
print "value at index=10 is ";value