Shelley invested $7000, part at 7% and the rest at 11%. If her total return for one year was $690, how much was invested at each rate?

My work:

So I thought I should add the 2 percentages together to make 18%

And then I don't know what to do.

Hmm... They were supposed to have covered this stuff in class, before assigning homework on it. It appears that they didn't....

To learn the basic terms and techniques, try

__here__. Once you have learned the basic terms and techniques, please attempt the exercise. If you get stuck, you can then reply with a clear listing of your thoughts and efforts so far. Thank you!

P.S. It's okay to use what you know from real life. You know that rates of speed don't add; for instance, if you drove 35 mph down one street and then 45 mph down another, you know that you never when 80 mph at all, let alone down both streets! If you pay 29% interest on the balances on each of four credit cards, you know that doesn't mean that you pay 116% on everything. Use this knowledge when interpreting word problems!