Set DrScheme's directory ? (HtDP)

Finally found this trick:

1. Start in a big language, like "Pretty Big".


2. Execute e.g.: (current-directory "c:/program files/plt/") in the bottom pane.


3. Then you switch to your lesson language (HtDP Beginnig Student for me) and reload (Run).

Found here: http://www.cs.utexas.edu/users/novak/drscheme.html

This allowed me to create in.dat so DrScheme could read it, allowing me to finish Exercise 2.2.1 in How to Design Programs (HtDP). Still no idea where it was looking for the file before I set that to my Desktop, lame..

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

Dear DIRECTV

Immediately remove premium channels from my account and refund erroneous charges. Please see my previous email on this subject in case there is any confusion about whether I ever wanted to pay for premium channels (hint: I did not). Note the change I made to my account package on the web last week, which inexplicably also did not remove unwanted premium channels which I had already erroneously been charged for.

Fix your web site so that customers can remove unwanted expensive programming without having to call you or use this form.

If I wanted to deal with this crap I would do business with the local cable monopoly.

Try harder.

ETA/b>: response: June 13, 2009 11:54:13 PM EDT
( unhelpful, if true )