He makes about 30 sales because he makes $100 typically but always makes a minimum of $25 making $75 worth of his sales. If you multiply 30 and $2.50 you get $75 in sales. His $25 minimum and $75 in sales make $100. So he makes 30 sales.
Photographing stars requires you to keep your exposure open for a long time. Taking pictures of star trails would take about 30 minutes to three hours. If you multiply that for the 6 stars stated above in the problem then you would be needing that much time if the stars were close and approximately 3 to 18 hours capture their star trails if they were far apart from each other.

Plug in -1 for x
and you'll get this:

Solve it further and you'll get:

Therefore,

where x = -1
You would start by 6÷3. This would tell you that for every proud of hamburger you pay $2.
Therefore multiply 10×2 and that equals 20.
Therefore, your answer is $20.