adric | So far today:

Conceived some trivial Scheme functions as part of working through HtDP through 3.0
Made coffee poorly.
Wrestled HP 3210 all-in-one (eg this) into functionality (printed, scanned) (This bout was a rematch.)
Discovered though HtDP 3.1 that I can no longer do simple algebra, as indeed I had previously suspected. That stings a bit.
Failed to ignore many bad things coming out from the telly.
Looked over last week's Time including interesting pieces on Twitter and Abrahamic monotheism.
Noticed that my trusty Ruby OSA scripts are giving Bus Errors.
Hunted up and pasted in images for the ef soundtrack mp3s I have assembled.
Determined that the era of crossposting to LJ has come and gone for me.

So, lazyweb, anyone want to explain to me how to find a function that calculates:

At a price of $5.00 per ticket, 120 people attend a performance. Decreasing the price by a dime ($0.10) increases attendance by 15.¹

I tried two equations in two unknowns, substitute one into the other to solve for one variable and promptly got myself lost. Much scribbling and calculations later I confirmed that I was way off. Help?

¹ is excerpted from HtDP 3.1