I recommend using a calculator, but working out the problem manually is alright too. Be sure to check your answer to make sure it's correct by looking over your work or by multiplying two of the numbers to see if it equals the third. The answer is about .92.
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Answer:</h3>
For each birdhouse built, the total cost increases by $3.50.
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Step-by-step explanation:</h3>
Let's try some of the answers and see if they work.
The "initial cost" will be the cost when 0 birdhouses are built.
... f(0) = 3.5·0 +24 = 24 . . . . . clearly, not $3.50
Then increasing the number of birdhouses by 1, the cost is ...
... f(1) = 3.5·1 +24 = 27.5 . . . . . the cost has gone up (not down) by $3.50
Clearly the total cost to build 1 birdhouse is not $3.50. (It is $27.50.)
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Our exploration of the formula leads us to conclude ...
... For each birdhouse built, the total cost increases by $3.50.