Week 11KenKen Y8

This is a preview of Parallel. You have to login or create an account, to be able to answer questions and submit answers.

Noun: Parallelogram Pronunciation: /ˌparəˈlɛləɡram/

  1. a portmanteaux word combining parallel and telegram. A message sent each week by the Parallel Project to bright young mathematicians.
  • Tackle each Parallelogram in one go. Don’t get distracted.
  • Finish by midnight on Sunday if your whole class is doing parallelograms.
  • Your score & answer sheet will appear immediately after you hit SUBMIT.
  • Don’t worry if you score less than 50%, because it means you will learn something new when you check the solutions.

1. KenKen

Today’s Parallelogram is dedicated to KenKen – a mathematical puzzle that is ten times more interesting than Sudoku.

First of all, these are the rules.

  1. You will be presented with a 3 x 3 grid.
  2. Each row and each column contain the numbers 1, 2 and 3 once and only once.
  3. Therefore, no number can be repeated in a row or column.
  4. Each marked region (or cage) contains a target number and a mathematical operation. The numbers in the cage must add up, subtract, multiply or divide to give the target number.
  5. The ordering of the numbers does not matter in a cage. So, 2 followed by 1 or 1 followed by 2 can both give the target (1,−), which means reaching the target 1 by subtraction.
  6. If the cage contains just 1 cell, the value of the cell is the same as the target number. it is a “freebie”.
  7. There are also 4 x 4 grids, and each row contains the numbers 1, 2, 3 and 4 once and only once. There are also even bigger grids.

This video summarises the rules of KenKen.

If you’re still unsure, this video goes through the rules one more time.

2 marks

1.1. Here is your first KenKen. The only mathematical operation is addition.

You can either copy it onto a bit of paper by hand or follow this link to open a copy of all of the questions that you can print and fill in.

To help us check whether you have completed the grid correctly, add up the 4 corner numbers and enter the sum in the box below.

Show Hint (–1 mark)
–1 mark

The (4 +) cage is one way to start cracking this KenKen. The cage has to contain (3,1), because the only other way to obtain 4 is (2, 2) and no number can be repeated in a row. We don’t know the order of the (3, 1) cage, but it tells us the remaining number in that column must be (2).

Correct Solution: 8

2. Two more KenKens

2 marks

2.1. What do the four corner cells add up to?

Show Hint (–1 mark)
–1 mark

The (3 +) cage has to contain (1,2), because it is the only way to obtain 3. We don’t know the order of the (1, 2) cage, but it tells us the remaining number in that column must be (3).

Correct Solution: 9

3 marks

2.2. This KenKen involves multiplication and division.

What do the four corner cells add up to?

Show Hint (–1 mark)
–1 mark

The (18x) cage has to contain (1,2, 3, 3), because it is the only way to obtain 18 by multiplying the four allowable digits. The repeated 3 is helpful, because they cannot both be in the top row, so one of them must be in the right column, middle row square.

Correct Solution: 8

3. Two tougher KenKens

4 marks

3.1 This KenKen involves only addition and subtraction, BUT it is a 4x4 grid!

What do the four corner cells add up to?

Show Hint (–1 mark)
–1 mark

Focus on the (7+) cage with only two cells They can only be (4, 3), but we don’t know in which order.

Show Hint (–1 mark)
–1 mark

Focus on the (3-) cage with only two cells. They can only be (4, 1), but we don’t know in which order.

Correct Solution: 10

4 marks

3.2 This 4x4 KenKen is even harder than the previous KenKen. Good luck. Don’t give up. There are two hints if you really, really need them.

What do the four corner cells add up to?

Show Hint (–1 mark)
–1 mark

Focus on the left column. The only way to fill the (12x) cage is with (4, 3), but we don’t know the order. Next, look at the (3−) cage, which must be (4, 1), but again we don’t know the order.

However, we know that (3−) cage has (4, 1), so the bottom left corner (part of the (12x) cage) cannot be 4, so it must be 3.

Show Hint (–1 mark)
–1 mark

In fact, we do know the order of (4, 1) in the (3−) cage, because the freebie (1) in the right column tells us that the (3−) cage must be 1-4, rather than 4-1, to avoid the two 1s being in the same column.

Correct Solution: 10

(This is your last KenKen... unless you have now caught the KenKen bug? Your teachers might want to introduce KenKen into the classroom, and you can find more for yourself by visiting kenkenpuzzle.com)

KenKen® is a registered trademark of KenKen Puzzle, LLC. Copyright KenKen Puzzle. All rights reserved.

4. Junior Maths Challenge Problem

4 marks

4.1 Which of these statements is true?

  • 15,614 = 1 + 56 − 1 × 4
  • 15,615 = 1 + 56 − 1 × 5
  • 15,616 = 1 + 56 − 1 × 6
  • 15,617 = 1 + 56 − 1 × 7
  • 15,618 = 1 + 56 − 1 × 8

In the context of the JMC we can assume that just one of the given options is correct, so we can find which it is by eliminating the ones that are wrong. We can do this by just considering the last digit (the units digit) of the given options.

The last digit of 56 is 5. Since 1 + 56 − 1 × 4 = 1 + 56 − 4 its last digit is the same as that of 1 + 5 − 4, that is, 2. So option A is not the correct answer. In a similar way, it follows that the last digit of 1 + 56 − 1 × 5 is 1, the last digit of 1 + 56 − 1 × 6 is 0, and the last digit of 1 + 56 − 1 × 7 is 9. So options B, C and D are also not correct This leaves E as the only possible correct option.

In this way there is no need to evaluate 56. However, to give a complete answer we would need to check that E is correct. This is straightforward:

1 + 56 − 1 × 8 = 1 + 15,625 − 8 = 15,626 − 8 = 15,618.

Before you hit the SUBMIT button, here are some quick reminders:

  • You will receive your score immediately, and collect your reward points.
  • You might earn a new badge... if not, then maybe next week.
  • Make sure you go through the solution sheet – it is massively important.
  • A score of less than 50% is ok – it means you can learn lots from your mistakes.
  • The next Parallelogram is next week, at 3pm on Thursday.
  • Finally, if you missed any earlier Parallelograms, make sure you go back and complete them. You can still earn reward points and badges by completing missed Parallelogams.

Cheerio, Simon.