puzzle square

I liked the puzzle posted by Jonathan, brought to my attention through Jo's resources

this puzzle involves one minus sign

for all pluses and all minuses between the boxes there are infinitely many solutions
but the one minus (a) and (b) and three minuses (c) versions lead to some interesting results

students can find solutions by trying out numbers and then adapting

resources (b) and (c) are intended as directed number practice

a powerpoint shows how algebra can be involved
(download it for the animations to work)