What number should go in the square marked X?

In the grid below, each of the blank squares and the square marked X are to be filled by the mean of the two numbers in its adjacent squares.
What number should go in the square marked X?
6 Comments
Simon Geard
7/3/2017 10:38:26 am
Labelling the blanks from left to right a,b,c there are four equations in four unknowns. For example
Reply
Simon Geard
7/3/2017 10:52:29 am
Another way of looking at it is as an arithmetic progression. In general
Reply
7/3/2017 12:05:45 pm
Simpler still, based on AP, is the fact that a gap of 30 (= 40 - 10) is covered in 5 steps so each gap is worth 6. You don't need to explicitly solve for T1 = a of the AP (as in Simon Geard v2).
Reply
Simon Geard
7/3/2017 12:47:15 pm
I think that is doing exactly the same thing it's just a bit disguised in your version. I also think that you need to show that the specification given in the question is equivalent to the sequence being an AP. If the question had said (mean+1) then the resulting sequence would be 10, 17, 22, 30, 36, 40 which is not an AP.
Reply
Simon Geard
7/3/2017 02:10:47 pm
Should have been
Paul
8/3/2017 05:58:19 am
I love reading these responses.
Reply
## Leave a Reply. |
## Puzzle Ideas
If you have an idea for puzzle of the month, then please do let me know. ## Archives
April 2017
Don't forget to check out 'Teach Further Maths' - PowerPoints for Teachers and Students of A-Level Further Maths ...and the brilliant NEW educational card game 'Maths Trumps' |

✕