Answer: 
This is the same as writing y = 650(0.907)^t
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Explanation:
Exponential equations can be of the form y = a*b^t
- a = initial amount
- b = growth or decay factor
In this case, we have
- a = 650 mg to start with
- b = 1 - 0.093 = 0.907 as the decay factor
If we had exponential growth, then we'd compute 1 + 0.093 instead.
Based on those values, we go from y = a*b^t to y = 650(0.907)^t which is the same as writing 
Other exponential forms are possible, but I think this form is the most intuitive. The 0.907 means that 90.7% of the sample remains after each year.
$21.50 - $4 = $17.50 If you start with $21.50, subtract the $4 entrance fee, you have $17.50 left to spend on rides. To figure out how many rides you can go on, divide $17.50 by $2.50. You can go on 7 rides. Here's the equation:
(21.5-4)/2.5=amount of rides
(21.5-4)/2.5= 7 rides
I think the first one might be 18 second might be 10
Four is greater than y, and -1 is less than y
