You find yourself playing a game of GreenJack with your friend.
It is played with a deck of only 16 cards, divided into 4 suits:
Red, Blue, Orange, and Green.
There are four cards in each suit:
Ace, King, Queen, and Jack.
All Aces outrank all Kings, which outrank all Queens, which outrank all Jacks, except for the Green Jack, which outranks every other card.
If two cards have the same face value, then Red outranks Blue, which outranks Orange, which outranks Green, again except for the Green Jack, which outranks everything.
Here's how the game is played: you are dealt one card face up, and your friend is dealt one card face down. Your friend then makes some true statements, and you have to work out who has the higher card, you or your friend. It's that simple!
Round 3:
You are dealt the Red Queen and your friend makes three statements:
My card could lose to a Blue card. Knowing this, if I am more likely to have an Ace or a King than a Queen or a Jack, then I have an Orange card. Otherwise, I don't. Given all of the information you now know, if I am more likely to have a Jack than an Ace, then I actually have a King. Otherwise, I don't.
Who has the higher card, you or your friend?
Hint
List all of the cards, and then eliminate some using (1).
Answer
Your friend.
Reasoning
You were dealt the Red Queen.
The possible cards, in order, are:
Green Jack
Red Ace
Blue Ace
Orange Ace
Green Ace
Red King
Blue King
Orange King
Green King
Red Queen (your card)
Blue Queen
Orange Queen
Green Queen
Red Jack
Blue Jack
Orange Jack
By (1), their card could lose to a Blue card (the Blue Ace), leaving:
Orange Ace
Green Ace
Red King
Blue King
Orange King
Green King
Red Queen (your card)
Blue Queen
Orange Queen
Green Queen
Red Jack
Blue Jack
Orange Jack
By (2), their card is not more likely to be an Ace or a King (6) than a Queen or a Jack (6), so their card is not Orange, leaving.
Green Ace
Red King
Blue King
Green King
Red Queen (your card)
Blue Queen
Green Queen
Red Jack
Blue Jack
By (3), their card is more likely to be a Jack (2) than an Ace (1), so their card is a King, leaving:
Red King
Blue King
Green King
Red Queen (your card)
Reasoning
We can multiply throughout by 12,
X X X X
- - - + - = -
X 2 4 12
to give:
12 − 6X + 3X = X
12 − 3X = X
12 = 4X
3 = X
As required.
Double-Checking
3 3 3 3
- - - + - = -
3 2 4 12
1 − 1.5 + 0.75 = 0.25
0.25 = 0.25, which confirms that 3 is correct.
?
Puzzle 112
Alex and Billie were rowing their canoe along the River Trent.
In the morning, they managed to row upstream at an average speed of 2 miles per hour.
They then stopped for a spot of lunch and a nice rest.
In the afternoon, the pace was a little easier as they were now rowing downstream back to their starting point, and managed an average speed of 4 miles an hour.
The morning trip took them 3 hours longer than the afternoon.
Hint
You can either fix the distance rowed and see what numbers work, or you can fix the number of hours.
Answer
12 miles.
In the morning, rowing at 2 miles per hour, they rowed for 6 hours. In the afternoon, rowing at 4 miles per hour, they rowed for 3 hours.
There are a number of ways of working this out, and here are two of them:
Method 1
If we assume the distance the rowed upstream to be D miles, we know the morning took (D ÷ 2) hours, and the afternoon took (D ÷ 4) hours, with a difference of 3 hours. So:
D D
- − - = 3
2 4
Multiplying throughout by 4 gives:
2D − D = 12
So:
D = 12 miles
They rowed 12 miles upstream.
Method 2
If we assume they rowed for H hours upstream, we know they travelled H x 2 miles in the morning. In the afternoon, they rowed for (H − 3) hours, and travelled (H − 3) x 4 miles. We know these distances are the same, so:
2H = (H − 3) x 4
Giving:
2H = 4H − 12
Rearranging gives:
12 = 2H
So:
H = 6 hours
They rowed for 6 hours upstream at 2 miles per hour, which is a total of 12 miles.
