In the short run (which is what this question is about), as output increases, the average total cost decreases where the marginal cost is below it. First you have to realise that increasing and...
Total cost divided by the times that cost has been paid for. For example, if the costs were 1, 2, 3 and 4 (of whatever currency), then the average cost would be 1+2+3+4 / 4 (because there were only 4...