r1 / r2 = sqrt(m2) / sqrt(m1) Notice the numbering system. If the 2 is on the denominator for the r, then it will be in the numerator for the m. You must substitute carefully.
r1 / r2 = sqrt(30) / sqrt(12)
r1 / r2 = sqrt(2)*sqrt(3)(sqrt(5) / sqrt(3) sqrt(4) but sqrt(4) = 2.
ri / r2 = sqrt(2)* sqrt(3)*sqrt(5) / sqrt(3) * 2 Now you can cancel out sqrt(3)
r1 / r2 = sqrt(2)*sqrt(5) / 2
r1 / r2 = sqrt(10)/2 <<<< answer
B <<< answer. Notice that they put sqrt(2) and sqrt(5) together again.