If you use the first four squares to set up a system of equations with the outer numbers as coefficents of 4 unknowns and the middle numbers as the RHS constant terms you can solve the system. Use this result find the value for 'a' missing number. I get -14.8421.
I doubt this is what the author is looking for.
Thanks for looking at it, I never thought of using this method. but your right, the authors are probably looking for a constant with no decimal places. By the way this puzzle was ment for 12 year olds!!
The answer is 4!
I used the first square to get the last square.
well that dose look pretty convincing, thanks a bunch dude!
I will be very disappointed if my answer is correct.