Answer to Q3. The stone weighs 10 lbs. of course. If one half of it weighs 5 lbs., then the other half must also weigh 5 lbs. Here's a little algebra for anyone interested (X = 5 + 1/2X).

Question #4 You're on a game show in which a grand prize is hidden behind one of three curtains,  Behind the other two curtains are pretty much nothing. You are allowed to choose one curtain and if the grand prize is behind that curtain, you win it! Now after choosing "Curtain #1" the MC opens up Curtain #2 to reveal to you that the grand prize was NOT  behind Curtain #2   and indeed must be behind either Curtain #1 or Curtain #3 (the MC, of course, knows where the prize is and will not reveal that curtain nor the one you picked).  Now the MC offers you the opportunity to change your selection from Curtain #1 and instead have Curtain #3.   Should you ?

a) Change your selection to Curtain #3.

b) Remain with your first selection, Curtain #1.

c) It doesn't make any difference, with two left it is a 50/50 chance on each.