Negative Numbers Puzzle

This puzzle is based on addition of negative numbers. Each white square (positive) cancels out one black square (negative).
In the first part, students have to find out the main rule. In the second part, they have to fill in the missing numbers which will describe each puzzle, sum them up and get the final answer.
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ⓘ To display the puzzle on a white board, please follow the link. To be able to add changes/edit, please make a copy of the Google Sheet.
ⓘ To reveal the answers, change the colour of a text on the right side from white to black.