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Add the numbers 1 through 9 into the grid so the calculations work out and equal the numbers off the grid. Note, BIDMAS/PIMDAS is ignored: A + B × C is the same as (A + B) × C, working left to right, top to bottom. Hint: a good place to start is look for lines where the answer is a semi-prime with a multiplication sign (one of the factors has to be on the side with one space).