It's embarrassing to say but I'm basically having to re-learn algebra after not touching the subject for about 5 years.

I'm going through ACT practice tests and this problem came up. The guide book helps you put the word problem into equation form but just assumes you know what to do from there to solve for X. And this one has been driving me crazy.

"Amy drove 200 miles at an average speed of 10 mphfasterthan her usual average speed. If she completed the trip in 1 hourlessthan usual, what is her usual driving speed?"

(r) is her usual speed.

Using the "time= distance/rate" formula, it guides me to come up with an equation of:

(200/r+10) = (200/r)+1

parentheses only there to distinguish fractions from integers

I know the answer, I've seen the key. It's 40. It's the process of getting to the answer that I can't figure out for the life of me, and it looks like it should be very obvious.

Best I could do was fill up half a page of scrap paper of breaking it out and breaking it back down again until I could "simplify" it down to r^{2}+10r=2000, but even from there I don't know what to do. It took way more time than anyone should spend on one problem during a timed ACT test, so I was wondering if someone could walk me through this, preferably with shortcuts. It would be greatly appreciated.