Hi There!
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We Know:
$35 per person.
Guides Fee: 10 * 35 = 350
425 - 350 = $75 (guide fee)
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Part A:
The problem states to put c as cost and p as number of people:
c = 35p + 75
b = $75 (guide fee)
If you don't need the one above, this is just in slope intercept form.
y = 35x + 75
b = $75 (guide fee)
Answer: c = 35p + 75 or y = 35x + 75
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Part B:
How much would it cost for 15 people?
Lets use: c = 35p + b
Substitute p for 15 people and b as $75 for the guide fee: c = 35(15) + 75
Simplify: c = $600
Answer: The total cost for 15 people is $600.
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Hope This Helps :)