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For example, suppose the fire is half the distance from the river that the fire-fighter is. Try that the other way around as well.
Suppose the fire is still half the fire-fighter's distance but both are now nearer the river than before.
Or further away.
Suppose the fire is still half the fire-fighter's distance but the two positions are further from each other along the bank. Closer along the bank ?
How are you finding the best position each time - by measurement ?Do one yourself with a calculator to help you see what calculation (formula) you need to make the spreadsheet do.