Question 20a.

You said that East should put the Queen of Clubs on the trick. You are correct. You may benefit in the end by the promotion of your Ten as a winner. If you do not put your Queen up, the Jack will win, and your opponents will still have the Ace and King of Clubs to play.

So the rest of this trick looks like this.
Dummy lead with the Jack of Clubs
East followed with the Queen of Clubs
Declarer plays the Ace of Clubs
West throws away the 4 of clubs.

Now Declarer has won 6 tricks out of the 8 promised in the contract. You can continue now to win your winners.

You are in Declarer's hand. So you play the Ace of Spades, to which West plays the 3, Dummy plays the 4, and West plays the 7.

You then play the 7 of Clubs, West plays the 9, Dummy plays the King, and East plays the 6.

You have made 8 tricks, which is your contract of 2 No Trump.

Whatever you lead now, you will lose, but you have a potential future trick in the Queen of Spades, so don't play it, but play in that suit, to see what happens.

So Dummy leads the 6 of Spades.

East's hand
Spades         K,10
Diamonds    5
Clubs          10

21.  What card should West now play?

a. King
b. Ten