We have a mathematical surd. Let's attempt it. As with all mathematical surds, there are solutions that come from many directions. We could express as a log, differential, integral, or polynomial.
Let's solve as with a polynomial.
r=10 IF r=7r
Canceling, we find:
If you choose 10=7r then you get r=10/7.
If you choose r =7 x 10 then you get r=70.
Neither one matches r=10 unless the r's are different.